Update admin.py

The problem is that always display primary avatar for User.
In admin page, the function get_avatar should return different avatar url for every avatar record
Merge pull request #18 from palmplam/master

Add an extra param to use user_id as userdirname instead of username
clean out
Merge pull request #19 from allenling/bugfix

Update admin.py (originally #77 )
README: Update name to django-avatar2 to avoid confusion
settings: Remove avatar/setings.py as it is no longer used