Add extra documentation for `column_formatters`

refs #3, fixed sqlalchemy.exc.DataError when saving empty AjaxSelectMultipleField

The mongoengine auth example won't work TypeError and AttributeError

Fixed error : raise TypeError(repr(o) + " is not JSON serializable") , TypeError: ObjectId('') is not JSON serializable

Fix Select2TagsField and Remove ModelView.column_hide_backrefs

Name all loggers.

Instead of using the root logger, give all used loggers names related to the package and their location.
This allows users to work with flask-admin logs as they wish, as well as being more clear where the logs are originating.

Fixed image re-encoding when uploading images. Thumbnails will keep transparency for transparent GIF/PNG files.

Inherit pk weak entities

flask_admin.contrib.sqla.tools.is_inherited_primary_key() now works even if the inherited primary key is itself a foreign key. This leads to a better support of SQLAlchemies *joined table inheritance*