Implement ycmd-minor-mode

Implement a minor-mode for ycmd. The mode adds hooks in order to have different triggers for a reparse. With the `ycmd-set-needs-parse` list the user is able to control when the buffer should be marked for reparsing. For now `save` and `new-line` or manual reparse is supported, but might be extended for other methods like an idle-timer. 
Ok, you're right. I will remove this function, since it doesn't bring any real advantages. One can just switch to the server buffer. If the server buffer wouldn't be always visible, it might make sense having such a function.