Issue 635 - Don't center-align cards in global style rule

[Issue 635](http://code.google.com/p/ankidroid/issues/detail?id=635)

Anki creates new note type style sheets using the ```text-align: center;``` rule by default. A user who wishes not to center-align cards can either change that to ```text-align: left;``` or simply remove the rule. In the latter case, AnkiDroid will still center-align cards because we have it defined as centered in flashcard_css.