Added client to handle the setup of stuff.

Added a client to handle the setup of stuff. It delegates #keyspaces, #keyspace, #[], and #batch to connection. 

Also provides reader methods if you want to re-use those in other ways.

You can now just do this and go to town, no need to bother with driver, executor and connection.

```ruby
client = Cassanity::Client.new

keyspace = client[:keyspace]
keyspace.use

keyspace[:column_family] 

# or
client[:keyspace][:column_family]
```

This seems way easier than it was before. I like it where it is heading.

Is this what you were thinking @technoweenie? Anything that could be easier?

/cc @derekgr