NEW FEATURE IMPLEMENTATION - MESSAGE STARRING

Regarding the implementation details, please, refer to [detailed documentation of the Message Starring feature's development](https://drive.google.com/file/d/0B3rfkCYLVzfGS1pldVYyT2daRG8/edit?usp=sharing) and [forum thread](https://bitmessage.org/forum/index.php?topic=3366.0).