cs-completer: improvements for solution detection

Second take on https://github.com/Valloric/YouCompleteMe/pull/886 .

I have ported the changes to the new ymcd-subrepo and merged them with the ones in the master branch there.  The detection of solutions is now in a separate ```solutiondetection``` module (feel free to rename it) as well, so in ```cs_completer.py``` the path for the solution to be loaded is retrieved through ```solutiondetection.Detect( request_data[ 'filepath' ] )```. I hope you had something like that in mind.
I'm not sure what you are expecting to get back from the `CSharpSolutionFile` function. Why is there an option of full path or something else? It should just be the full, absolute filepath to the solution the user wants loaded, nothing else.