Added downloads badge
Testing resources
Added `shiva.utils.parse_bool` helper

It parses the string 'false', '0' and '' as false values. Any other value is
evaluated as true.
Testing tracks
Merge pull request #129 from tooxie/tests

MOAR TESTS!
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`.
[WIP] Added PUT method