Allow fetching of a DriveFile object by NodeRef

To avoid users of the GoogleDocsService having to know too much about
the underlying Alfresco datamodel being used, provide a method to
fetch the DriveFile object by NodeRef, and update existing code to
use that.

Closes Alfresco/google-docs#3
Use a specific Exception for nodes with no Resource ID

Within the Google Docs Service, there were lots of occurances of

        // TODO Wrap with try for null

This defines a specific exception, NotInGoogleDriveException, which
can be returned when attempting to operate on a node with no Resource
ID defined, and updates the service to throw this in that case
Merge pull request #4 from Gagravarr/master

Allow fetching of a DriveFile object by NodeRef