Major changes to the User Framework, SinkCommands: add /message, add Travis CI Integration, fix some minor bugs

User Framework:
Add an API for adding custom SinkUser instances for custom CommandSenders. This allows third party plugins to integrate better into SinkLibrary. Add SinkUserProviders which will handle some stuff for the SinkUser classes. E.g. load/unload them, create new instances, etc... This also allows better handling of other SinkUser classes like IrcUser at the StringUtil class.

Minor Bug fixes:
Fix /sinkversion Commnad causing a NullPointerException

New Commands:
Added /message for SinkCommands, which e.g. allows better cross private chat between IRC <-> Ingame
-> Remove IrcPrivateMessageCommand
-> Remove MsgCommand

TestConfiguration:
Add comments to yaml paths

Misc:
Rename PlayerNotFoundException to UserOfflineException
Add UserNotFoundException
Rename UserConfiguration to IngameUserConfiguration
Add Identiefable interface
Rename SinkUser to IngameUser
Rename SinkIrcUser to IrcUser
Rename SinkConsoleUser to ConsoleUser
Remove the IUser interface, add SinkUser abstract class instead