Fix timezone handling and date parsing.

I use this test script to see that all parsing is happening correctly:

<?php
 date_default_timezone_set('UTC');
$dates = array(
  "1999",
  "1999 Jan",
  "1999 Mar 6",
  "1999 Mar-Jun",
  "2005 Nov 21-27",
);
foreach ($dates as $date) {
  apachesolr_biblio_format_value($date, 'date');
}