Modified cleaning code to remove NA from Embarked
The orginal code threw warnings as we tried replacing NA by the mode on a panda copy.
The code has now been modified to avoid this warning, following a trick on
stack overflow:
http://stackoverflow.com/questions/20625582/how-to-deal-with-this-pandas-warning
Merge pull request #1 from rousseab/master

Modified cleaning code to remove NA from Embarked