Ability To Specify Connection Timeouts

### The Problem

At this time it is impossible to set a socket-level timeout in cqlengine. This can produce problems when attempting to connect to a non-existent or downed node. By default the Python `socket._GLOBAL_DEFAULT_TIMEOUT` is used and this tends can be an extraordinarily long time for a database socket to wait.

### The Solution

The python `cql.connect` method takes an optional transport object. We are now manually constructing a `thrift.transport.TTransport.TFramedTransport` object so that we can set the timeout on the underlying socket object as this option is not provided by the cql library itself.

One transport is created per open connection and closed by `cql` once the connection is closed or the object is cleaned up by garbage collection.