Add nose plugin entry point to allow running BDD tests via normal nosetests client

[tysonclugg]

The changes to setup.py allow nose to find and use the plugin directly from the normal nosetests command line client, rather than requiring use of the special aloe client. I'd prefer that we didn't have to pass --no-ignore-python and that other tests ran by default, but that may be the subject of a different PR.

[koterpillar]

I'm happy to make --ignore-python default for invocation as aloe only, not nosetests if you'd want to add that too.

[tysonclugg]

I wasn't sure you'd be comfortable with the changes in #115, you may pick which one you wish to merge...

[tysonclugg]

I feel like #115 is a bigger task that will be embroiled with questions and side issues. Please, can this PR be accepted now so it doesn't remain sidelined whilst #115 is decided?

It's a simple change with two big benefits:

• Can run all tests in one command.
• No need to combine coverage/xunit output files from multiple test runs.

[koterpillar]

Makes sense, accepting this.