Removed old 'base' module and create rev_auth and rev_model_auth modules

(no functionality in either yet!)
Module Loader now loads the model classes of installed modules

The system now automatically imports RevModel classes from any python
files in the 'models' directory of a module, which saves the user from
having to remember to add 'import' statements to __init__.py (a blank
__init__.py file is still required though)

Also a couple of other design decisions made:
- Model names should be singular
- Removed requirement for manually specifying the '_name' for the
database. We now just take the capitalised class namd and split it with
underscores