Added mechanism to predefine associated macros for extensions. Also refactored the way extension behavior is initialized and updated.
Please note that I still need to add validation that appropriate extensions are enabled before using an extension function.
BUG=25
Review URL: http://codereview.appspot.com/2139042

git-svn-id: https://angleproject.googlecode.com/svn/trunk@413 736b8ea6-26fd-11df-bfd4-992fa37f6226