Added relationship tables

Relationships are many to many between track <> artist, and track <> album.
This means that there is no artist <> album relationship any more. There is,
however, a calculated attribute `Artist.albums` and `Album.artists`.
Tests for PUT method

It could use some more corner cases, but it's good enough for a start.
PUT method

We add the possibility to update our models using the PUT method.
Running tests wiped out the main DB

Fixed bug which wiped out the main DB after running the tests.