Added an rmgpy.utilities module with path_checksum() function.

This function returns the checksum (hash) of a list of files/folders,
eg. 
  hash = rmgpy.utilities.path_checksum(['path/to/database'])

This will be useful for checking whether things have changed, for cache validation.
Cache the RMG Database in a pickle and a hash, to speed up reloading it.

We hash a bunch of metadata to try to be sure that the cached database is the
same as what would be loaded if you loaded it from scratch. Hopefully I have
included everything that matters.
Use scoop.shared module to share database cache location and hash.

We pass around the location and hash of the database cache file so
that each worker can load it and check it has the right version.

This removes the need for the "RMG_DB_FILE" environment variable.
Tweak to scoop message when loading database.

Not sure where these get logged to, if anywhere, for the remote workers.