Add more functionality and make API easier to use

[pyko]

Note: This is a breaking change as I've modified the API, but it should now be easier to use :)

Added support to allow:

• sign out
• show achievements
• show specific leaderboard
• increment incremental achievement
• check if user is logged in or not

Also just general cleaning up of code

[ptgamr]

Thanks @pyko for your contribution!