Add RobustROS estimation method

Closes #42

 - Currently does not have a plot method. Currently considering just adding a comparative QQ Plot per the notebook in #42. 
 - Test class currently relies on nosetests. Probably should look into just using the built-in unittest module like the rest of the code.
 - Should probably add one or two more tests datasets from literature.
 - My text editor trimmed some trailing whitespace from other parts of the file. Sorry about the resulting noise in the diff.