Implement "jumping to definition"

This is related to #151. I implemented the functionality on irony-server. I think this functionality is fairly easy to implement using clang library, and the implementation is not ugly. The performance may be a bit lower than those indexing versions, but it's not critical -- and it saves time to set up indexing daemons. So, in my opinion, there is no need to wait for another "indexing framework" to implement this.

Usage: irony-server find-definition file-path line col

Output: ("path-to-file-with-definition" line col)