Create configuration and database routing

[EliuX]

This PR:

• Upgrades pytest to version 5.2.0, so we can use pytest-mock.
• Tries out a mock in time_enties_namespace_test.py
• Add support for configurations. By default it works with time_tracker_api.config.DefaultConfig.
• Creates a configuration for a whatever database that can be taken as an example of what the access to any database should look like in a design/architecture that can be extensible.
• whatever_repository.py how any database implementation (MongoDB, SQL databases, etc.) can be easily hooked using a generic Repository pattern.

In addition, I added a refactoring of the API models for what we thought would be a better naming system.

[Angeluz-07]

Could we have type hints here?
Also, check the name of the file, I think It should be 'time_entries_namespace_test' or why not just 'time_entries_test' ?

[EliuX]

Ok

[Angeluz-07]

Could we have type hints here.

[EliuX]

Sure

[Angeluz-07]

Why not call this function 'init_database' directly ?

[EliuX]

There is some sort of standard that some objects like the APIs in flask Restplus have a function called init_app so they take the Flask app as a parameter and they do whatever initialization logic they need to be ready to be used by that app. So whatever module or package you add if you want to make it like pluggable to Flask this is pretty much the way to go