Add a .travis.yml file for continuous integration with travis-ci.

Travis-ci is a hosted continuous integration solution for the open source community.  It is without a doubt the largest and most successful such initiative and it's really easy to set up.

Once this pull request is merged and travis-ci is enabled for the rackspace/pyrax repo (which will require a member of the rackspace organization to sign in to https://travis-ci.org/ configure it) all pull requests submitted against pyrax will get built and their commit status on github will be updated.

People who fork pyrax will also be able to enable travis-ci on their own repos and get continuous integration as they're developing their contributions.

Here is an example run from my own repo: https://travis-ci.org/dreid/pyrax/builds/5290094

I've enabled only python 2.7 and pypy and I've marked pypy as an allowed failure.  It didn't look like you had a preferred test runner so I've used nosetests.

I also am only running the unit tests but it should be possible to use [Secure environment variables](http://about.travis-ci.org/docs/user/build-configuration/#Secure-environment-variables) to setup the necessary resources for the integration tests.

Once this is merged and travis-ci is enabled for this repo you can also add a markdown [status image](http://about.travis-ci.org/docs/user/status-images/) to the README.