Makes the vibe.d version fully work, cleans up/extends the public API and prepares for publication on the DUB registry

I've *finally* gotten a little bit of time to start playing with Cassandra and have worked a bit yesterday to extend the library.

The main changes are:
 - Fully works now with and without vibe.d
 - "package.json" has been adjusted to be viable for code.dlang.org
 - `CassandraKeyspace` and `CassandraTable` have been added
 - A lot of public declarations have been made private/protected to avoid misuse
 - "dub --build=ddox" produces useful documentation

I'd also like to go through all names and switch to either Phobos or vibe.d naming convention as it currently is still a bit inconsistent in some places, but wanted to get your opinion on it (and the existing changes) first.