ServerOptions class

[flowersinthesand]

As pointed in cettia/cettia-protocol#12, introducing a dedicated class to represent options about server, i.e. ServerOptions, would be better in Java as well.

[flowersinthesand]

On second thought, it seems to be excessive to introduce a new class and create relationship between other classes just for one option, i.e. heartbeat. It would be wise to wait for the feedback regarding this issue.

I'll remove it from the milestone. Sorry for the confusion.