Gems updated/added.

Added YARD and Guard, updated Rspec.
Refactored Client to be thread-safe.

It used to keep the API key in a class variable, which was very thread
unsafe at it's shared across every instance of the Client.

Also, moved errors to their separate file, and Client is now defined in
lib/rmeetup/client, while lib/rmeetup only requires everything.
Updated readme (docs).
Guardfile added.