Add a basic tox.ini file (fixes #6)

I've added a basic tox.ini which:
* runs the tests using trial under python2.7
* prints a coverage summary
* prints a pyflakes report

Also added a few paragraphs to README.rst explaining how to run the tests.

The tests are currently `trial` specific, otherwise I'd have used py.test
And since `trial` hasn't been ported to python3, I can't attempt to run the tests under python3.4. 
See #16 for removing twisted dependencies.

Once this is merged I can create a travis recipe to run the tests and pyflakes, automatically via tox. See #5 

And  once the tests are running via travis, I can get coveralls running. See #15.

Fixes #6 
This is a pet peeve of mine.  It is either tox's job or the reader's operating system's job to teach the user how to get a working tox installation.  Maybe other people think it is appropriate to give instructions like this, I dunno, so I'm not really objecting here.  But consider that this is a greatly oversimplified explanation of how to get a working tox installation.  Anyone who actually needs to be given this information may just get themselves into trouble by following it.  And of course anyone who doesn't need it... doesn't need it.