Put charset back into kwargs

Moving charset out of kwargs creates an issue with precompile in the base.py.

TypeError at /admin/widgy_mezzanine/widgypage/add/
__init__() got an unexpected keyword argument 'charset'
Request Method:	GET
Request URL:	http://localhost:8000/admin/widgy_mezzanine/widgypage/add/
Django Version:	1.6.5
Exception Type:	TypeError
Exception Value:	
__init__() got an unexpected keyword argument 'charset'