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.
database.py needs to import some functions from util_validation. If they are are util.py instead, then datbase.py would need to import from util.py. However, inside util.py we import something from database.py, causing a cyclic dependency. The separate util_validation module will make the process of separating util.py easier in the future so it should be welcomed, I think.