Python 3.3 compatibility

This PR makes the necessary changes so that cartopy works with both Python 2.7 and Python 3.3. Use of the lightweight third-party six module (http://pythonhosted.org/six/ - installable via pip) means that 2to3 is not needed.

In a couple of places I've simply replaced ```iteritems``` with ```items``` and ```filter(...)``` with ```list(filter(...))``` etc. This may have a minor performance impact but the tests appear to run ~1 sec slower which might be in the noise. There is the option of using ```six.iteritems``` etc. if this is deemed unacceptable.

When running the tests under python3 under osx I get 16 test failures (compared to 12 under python2.7), but these failures are all minor image differences, small numerical differences plus a proj4 difference related to Robinson. All tests passed locally under python2.7 on linux.