Drawing distinction between prefix and path_prefix.

- ``layout.prefix`` refers to the full URL prefix for a resource.
- ``layout.path_prefix`` refers specifically to the path portion of the
  prefix.
- Moving to ``os.path.join`` in place of straight string interpolation
  in the few places interpolation was used for URL construction.
Fixed call to wabs.uri_get_file
Fixed signature of blobstore.wabs.CallingInfo.connect.
Merge pull request #57 from socrata/fixup_55

Fixing up a few bugs introduced in last refactor.