Add unit tests

[SamyPesse]

• handleTab (First tests #13)
• handleReturn (Test handleReturn #14)
• handleKeyCommand (Unit test handleKeyCommand #15)
• handleSelectionInBlock (Test hasSelectionInBlock #16)
• getKeyBinding (Test getKeyBinding #17)

[mxstbr]

Done, all of the public API is now unit tested with 100% test coverage!