Brownju/bug/47

Added a module for validating user input, moved the location of orvsd_central.database.Model to orvsd_central.models.Model, and added tests to general.register() which the function itself still needs to be tested for correctness.
Extrapolate this into:
```
try:
    user = User(...
        ...
    )
except IntegrityError:
    # check for unique email
    # check for unique username
    # tell the user which one wasn't unique
```