A base class for server-side exceptions to be grouped by

This is important if you want to detect if there was an issue that resulted from the server being broken specifically, as opposed to the client issuing a bad command.

See: https://github.com/disqus/nydus/pull/31

Also, let me know if there are other exceptions that could extend the base `ServerError`. These are just the two that stood out.
Merge branch 'master' of git://github.com/mattrobenolt/redis-py into mattrobenolt-master

Conflicts:
	redis/exceptions.py