Added an Echonest wrapper (#7)

Added a class that conveniently wraps the Echonest API.
Changed schema (#7, #9)

Added a songs table and moved several columns for database
normalisation.
Adapted some column types to PostgreSQL (SERIAL, TIMESTAMP).
Integrated the Echonest wrapper (#7)

Integrated the Echonest wrapper in existing code.
Added models, refactored code

Added new models that reflect the new schema and also refactored code.
Added a Soundcloud wrapper

Added a class that conveniently wraps the Soundcloud API.