Fix compat for CPython/PyPy 2.x

Support both __builtins__ as a dictionary and a module.

Allow custom _id value type in mongoengine backend

Display foreign keys when explicitly instructed

__builtins__ should be accessed as a module, not a dict.

More examples

use a macro for generating admin static urls that can be easily replaced

this makes it easy to re-use the builtin templates when the Admin is bound to
alternative endpoints (other than "admin")

Fixed #379. Redirect using request.url when using "save and continue" in model edit_view

this will work for apps located at places other than the server root

Fix counterintuitive editing of MongoDB Documents with ImageField

form.js - make fieldConverters usable