Add table namespacing

[ob-ivan]

There are three modes application can run:

• production mode
• development mode
• testing mode

Each mode requires its own set of tables in database to run on.

Currently production tables are named as is, e.g. exercise, while development tables are prefixed with _test_, e.g. _test_exercise.

It would be much cleaner if each mode had its own prefix defined, like following:

• prod_ for production
• dev_ for development
• test_ for testing

[ob-ivan]

Done.

[ob-ivan]

Just referencing #139 to let it know this issue is closed now.