(cmake) Rename version.h.in into config.h.in

Add 2 constants for Beatport API key & secret
(o2) Simplify lib building
(cmake) Fix some compile problems with includes and dependencies
(oauth) Use KEY and SECRET values to connect to beatport API
(misc) Add a singleton interface
(oauth) Do not instanciate O1Beatport at startup
(singleton) Allow mutex usage in subclasses