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.
Defining imports in functions is how you get around circular imports. It's better to architect around it if possible, but when it's either too time consuming or infeasible then you just define the imports in the function that uses them. In this case it'd be ``from orvsd_central.util_validation import (prompt_valid_email, prompt_valid_username, prompt_valid_password`` inside of ``init_db``.