Added a validateArguments function to throw meaningful error messages and added testing framework.

Hi!  One thing that caught me when getting started with Chart was that it would bomb if a valid animationEasing option wasn't passed into the constructor so I just added a simple function to check that a valid option was added.  

I also added a unit testing framework (mocha & chai) to validate that the method worked as expected.  Now you could require that new pull requests have tests to make sure they are not breaking stuff  :-)