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.
Due to the above comment, I'd recommend making it look something like this:
```
unique = User.query.filter_by(username=username).first()
```