Tighter Django integration

[gunthercox]

This makes changes so that if ChatterBot is being used with Django, the Django Statement and Response models can be used in place of ChatterBot's Statement and Response objects. Previously the storage adapter converted incoming statement objects into Django model objects before saving and/or returning them. Now this conversion is no longer required.

This is going to need further testing before it gets merged.

Items that need to be checked off before this can be merged:

• Make sure ChatterBot statements/responses and the Django models have the same methods
• Make sure logic adapters all work with in Django
• Make sure input adapters all work in Django
• Make sure output adapters all work in Django

Closes #464

[vkosuri]

@gunthercox Any plan to fix this issue #308

[gunthercox]

@vkosuri I'm planning on taking care of #308 in a pull request after this one gets merged. I'm pretty sure the "reversal" is just an issue with confusing variable names.