PEP8 and cosmetic changes
Prefixed the ALLOWED_ORIGINS config
Renamed helper.py to decorators.py
Moved DEFAULT_ARTIST_IMAGE and DEFAULT_ALBUM_COVER

Moved those 2 constants to the config file. Made no sense to have them in the
resources.py file. As a side effect, the resource_fields attribute had to be
turned into a method because the app would be outside of the request context
otherwise.
Added CORS_ENABLED setting
Changed CORS_ALLOWED_ORIGINS accepted values
Created shiva.http module
Created shiva.mocks module