Add basic Travis CI setup with test script that runs JSHint

I'm hoping this will be accepted as it can be a starting point for adding more automated tests including JSHint and QUnit. Travis CI also has the ability to run PhantomJS so there is a possibility of running more sophisticated UI tests. Of course, the benefit of CI is that we build and test automatically with each commit!

If the pull request is accepted, all you have to do is follow steps 1 and 2 in the Travis CI ["Getting Started" document](http://about.travis-ci.org/docs/user/getting-started/) to setup the flot/flot repo. I've already included the .travis.yml configuration file in the pull request. You should then be able to see a (failing!) build at https://travis-ci.org/flot/flot and the status image that I added to the readme should light up. 
For example: https://travis-ci.org/brianpeiris/flot

Additionally, you can now run JSHint locally by running the following in your command line:

    npm install
    npm test