Add drush backend

The benefits include
- decreased burden in setup: it has a drush.bat for windows; and for linux, all you have to do is ln /usr/bin/php to drush.php
- its' already ported to D7 !!
- it's very close to being, or already is core-worthy
- many people already use it and are familiar with using it at the command line. this project could greatly extend its user base

@Frando: as per your question in IRC about WordPress Automatic Upgrade, I found on its faq: http://wordpress.org/extend/plugins/wordpress-automatic-upgrade/faq/

Do I need to change permissions on files?

No WPAU will ask you for your FTP credentials and do it automatically.

I'm setting this back to active because Frando's "drush pluginmanager apply" idea hasn't had a chance for evaluation. This also relates to running the command on cron. Here's catch__ and chx's discussion about running this on cron:

 <catch__> if we can queue stuff and have drush run via cron, that'd be pretty cool.
 <chx> because we created an automated mechanism again where you do something on a webserver and automatically files get written and eventually executed by Apahce
 <chx> not immediately
 <chx> but eventually
 <chx> so this needs quite some thought
 <catch__> chx: not by apache, run via php cli.
 <catch__> chx: extra cron job just for cron.
 <catch__> just for drush.
 <chx> catch__: there is a chain of actions here. you tell pm , i want this installed. cron runs , php file gets written. so if i can do a CSRFC attack against PM then you are toast...
 <chx> *CSRF
 <chx> so while Apache does not write PHP, indirectly it does
 <catch__> chx: yes of course - and ftp handles this by requiring user/password right?
 <chx> yes
 <chx> and password is not stored
 <chx> now, if you go indirect route then you must store
 <catch__> chx: OK, so seems like a solvable problem, just needs to be solid.
 <chx> not easy
 <catch__> chx: well, settings.php?
 <chx> I am not sure it's solveable