Adding InlineFormsetMixin for working with inline formsets simply

I have to be honest, I haven't had a chance to test this properly yet. I plan to do so within the next couple of days, but I wanted to start the discussion early in case anyone with a little more experience might be able to do a better job of it.

The idea of this mixin is that it's sometimes a pain to create ModelForms in an easy, generic manner with multiple other models that have forms needing to be rendered at the same time. So this mixin allows you to specify `inline_formsets` in your class-based view, and then the template's context data will be augmented to include the inline formsets as extra entries to the context dictionary. In addition, `form_valid()` is overridden to check the validity of the formsets before saving the main ModelForm and the corresponding inline formsets.

This was inspired by this blog post: http://haineault.com/blog/155/

P.S. This is my first time working with reStructuredText, and all of the other concepts are a bit scratchy in my mind at this point. Please forgive me if I ended up screwing the whole thing up.