Code review comment for lp://staging/~leonardr/lazr.restfulclient/shorten-cache-filename

This branch fixes bug 512832 by ensuring that the filename of a cached representation is never longer than 150 characters. The filename always ends with an MD5 sum derived from the resource's full URL, so truncated filenames won't collide unless there's also a hash collision.

This code copies-and-pastes in code from httplib2. I filed an httplib2 bug (http://code.google.com/p/httplib2/issues/detail?id=92) to deal with the underlying problem in such a way that I can eventually get rid of the copy-and-pasted code.

Tests make up most of this branch.