Extended OfflinePlayer API

[YouHaveTrouble]

• Added some methods to OfflinePlayer class.
• Added convienience method for saving player file after modifications to the nbt data
• All methods fall back to getPlayer methods if the player is online

edit: apparently build failed, going to bed, gonna fix when I wake up
edit2: builds and runs for me locally

[katekerllenevich]

lgtm, waiting for billy.

[YouHaveTrouble]

There's a small discrepency between the javadoc and impl, fixing in next commit