Added new Follow endpoints

https://github.com/jwilsson/spotify-web-api-php/issues/17
Added tests for new Follow endpoints

https://github.com/jwilsson/spotify-web-api-php/issues/17
Add examples of Follow endpoints

https://github.com/jwilsson/spotify-web-api-php/issues/17
Merge pull request #18 from vdesabou/master

Added new Follow endpoints