Pytest integration tests

I have been sitting on this for a while. It is a port of @KennethNielsen's integration tests over to the pytest framework.

You can now run ``py.test`` and all tests that do not require hardware will be run. If you want to test hardware integration, pass an ip address, like this:

``py.test --ip 192.168.1.101``

If you have installed the pytests coverage plugin (https://pypi.python.org/pypi/pytest-cov) you can generate code coverage reports like this (for example):

``py.test --cov-report term --cov=soco/core.py ``

Of course, you can do a lot more with the pytest coverage tools, such as generating pretty HTML reports.

There are still a couple of tests to fix up, but I would appreciate any feedback at this stage

Lawrence