simplify GoogleDriveFileRevisionMetadata
simplify GoogleDriveFileRevisionMetadata

 * GoogleDriveFileRevisionMetadata inherits from
   GoogleDriveFileMetadata, yet was reimplementing many of its
   methods. This commit removes identical methods from the child
   class, factors out the filename property so that the `name`
   and `export_name` methods can use the same base implementation,
   and adds docstring to explain some of the provider quirks.

CenterForOpenScience/waterbutler,RCOSDP/waterbutler,TomBaxter/waterbutler,rdhyee/waterbutler,felliott/waterbutler,Johnetordoff/waterbutler