Added new Follow endpoints

https://github.com/jwilsson/spotify-web-api-php/issues/17

This is what I came up with.

Some examples:

```
		$api->currentUserFollows('artist', '74ASZWbe4lXaubB36ztrGX,2t9yJDJIEtvPmr2iRIdqBf');

		$api->followArtistsOrUsers('artist',array('74ASZWbe4lXaubB36ztrGX','2t9yJDJIEtvPmr2iRIdqBf'));

		$api->unfollowArtistsOrUsers('artist',array('74ASZWbe4lXaubB36ztrGX','2t9yJDJIEtvPmr2iRIdqBf'));
```