Add keystroke to jump between the header and the corresponding source file.

Inspired by ff-get-other-file section from
https://code.google.com/p/chromium/wiki/Emacs

And simplified from:
superuser.com/questions/255510/how-to-toggle-between-cpp-and-hpp-that-are-not-in-the-same-directory

Signed-off-by: Thiago Farina <tfarina@chromium.org>