Fixed deadlock that occurs in an import scenario.

When user code imports code that initiates a new connection, it could cause a
deadlock when String.encode was called in a seperate thread (e.g. when reading
messages from the cassandra server).
See http://bugs.python.org/issue10923

Reproduce:

connect.py:

    from cassandra.cluster import Cluster
    cluster = Cluster(["127.0.0.1"])
    session = cluster.connect()

prompt:

    $ python
    Python 2.7.3 (default, Apr 10 2013, 06:20:15)
    [GCC 4.6.3] on linux2
    Type "help", "copyright", "credits" or "license" for more information.
    >>> import connect
    (stuck)

Ubuntu 12.04.3 LTS running in a VirtualBox