Added a practically usable example ycm_extra_conf.py for json db.

Hi Valloric,

Probably creating a pullrequest is not the best way to communicate with you, but I didn't know how to get in touch with you. Probably this layout/structure of this pullrequest is not the best fit to your project.
Anyway, I have created a more sophisticated .ycm_extra_conf.py which could be useful for folks who are really dependant on the compilation json db. 
The ycm_extra_conf.jsondb.py is for finding flags in the compilation database for headers whose counterpart source file is not existent.
This is done by browsing the json db to find one source file (a translation unit), which uses the directory of our header as an include path (-I, -isystem).

Hope you and others might find it useful, and thanks for this awesome plugin,
Gabor