Added average answer time (revlog) to internal columns.

A recent Anki-users post asked how to identify cards which took too long to answer.[1] I thought of hssm's addon. It looked very easy to add this feature. Thanks!

http://groups.google.com/group/anki-users/browse_thread/thread/13920eba8fb6092c?hl=en#
Merge pull request #1 from someguy2013/master

Update internal_fields_in_browser.py