PoHeader::__construct() throws warning and breaks localized installs

@catch: i'd be happy to tweak the comment format but afaict from https://drupal.org/node/1354 i'm already doing it right. i guess i'm missing something; could you let me know how you'd like me to write that comment?

thanks for the tip, Cottser.

catch: as i mentioned above, this gets run every time a .po file is loaded, which happens early during site install before we've even asked the user what their timezone is. in fact, this needs to run so that we can ask the user that question in their own language :) once they've answered that question, drupal calls date_default_timezone_set() for us and all's good; it's a temporary problem (but one that breaks site installs for some setups). we could use try/catch to set this temporarily to UTC if no timezone's been set yet once #1247666: Replace @function calls with try/catch ErrorException blocks lands, as you suggested earlier, but for now i don't see an alternative other than to ignore the warning.

gábor: 'd8mtl' was a tag we asked people to use during a D8 code sprint in montreal in january (http://groups.drupal.org/node/272733), which was well attended, although most people didn't bother to use the tag. if that's not how we're supposed to use tags, let me know and i'll remove it from this and the other issue.

ini_get() doesn't help because date_default_timezone_set() does not do the same thing as ini_set('date.timezone', ...); php keeps those separate (see below for a demonstration). we would have to call @date_default_timezone_get(), check $php_errormsg, then set a temporary timezone if there was a warning. that doesn't seem like a big win to me.

mvc@formation:~% php -v
PHP 5.3.21-1~dotdeb.0 with Suhosin-Patch (cli) (built: Jan 27 2013 10:27:28) 
Copyright (c) 1997-2013 The PHP Group
Zend Engine v2.3.0, Copyright (c) 1998-2013 Zend Technologies
    with Suhosin v0.9.33, Copyright (c) 2007-2012, by SektionEins GmbH
mvc@formation:~% php -a
Interactive shell

php > print date_default_timezone_get();
PHP Warning:  date_default_timezone_get(): It is not safe to rely on the system's timezone settings. You are *required* to use the date.timezone setting or the date_default_timezone_set() function. In case you used any of those methods and you are still getting this warning, you most likely misspelled the timezone identifier. We selected 'America/New_York' for 'EST/-5.0/no DST' instead in php shell code on line 1
America/New_York
php > print date('r');
PHP Warning:  date(): It is not safe to rely on the system's timezone settings. You are *required* to use the date.timezone setting or the date_default_timezone_set() function. In case you used any of those methods and you are still getting this warning, you most likely misspelled the timezone identifier. We selected 'America/New_York' for 'EST/-5.0/no DST' instead in php shell code on line 1
Fri, 22 Feb 2013 11:16:06 -0500
php > $a = ini_get_all();
php > print_r($a['date.timezone']);
Array
(
    [global_value] =>
    [local_value] =>
    [access] => 7
)
php > date_default_timezone_set('America/New_York');
php > print date_default_timezone_get();
America/New_York
php > print date('r');
Fri, 22 Feb 2013 11:16:47 -0500
php > $a = ini_get_all();
php > print_r($a['date.timezone']);
Array
(
    [global_value] =>
    [local_value] =>
    [access] => 7
)
php >

right, it just gives up and throws a warning instead of trying to guess -- that's php for you. anyways, thanks for the commit!