Allow to pass in the Google JSON file with the credentials on the command line

Instead of patching the file, it's nicer to allow passing the
path to the JSON credentials on the command line.
Output an error message if the user did not patch the client_id

This will also output an error message if we did not find the client_secret
or client_id from the credentials file.
Try to find the Google credentials JSON file(s) in the user home directory by default

This basically makes the setup work without needing to patch any file
and also without needing to pass any parameter on the command line
if the user just saves the file from Google to their home
directory.
Add ->search() method and documentation for non-traditional file search
Merge pull request #6 from Corion/master

Add direct support for the JSON file downloadable from Google