v2.0.0.rc0

First release candidate of v2.0. Unless any bugs are found in the next few days this is going to be v2.0.

• Authentication has moved to its own namespace, Cql::Auth. If you have implemented your own authentication this is a breaking change.
• Lots of documentation updates
• Added #last_page? and #next_page to QueryResult and VoidResult to make the API the same regardless of whether the concrete result is a a void result, a normal rows result, or a paged result.
• Removed the docs for some deprecated features. They still work, but are no longer part of the public API.
• Fixed #94, a silly bug where giving an empty list of hosts to Client.connect failed with an unrelated error.