Fixing get for admin field instance, and adding proper tests.
Removing unused imports.
Fixing and testing delete for admin field instance handler.
Cleaning up BaseHandler.

* Sort imports in basehandler.
* Use str.endswith instead of slicing.
Remove unused functions and unused args fom field handler.
Stronger validation on field type.

Define the proper regex for dealing with the different field types.
Populate TestAdminFieldCollection tests.

Actually test get() method of AdminFieldCollection.
Documenting api in admin/field.