Py2to3

All files in the `quantecon` package should be python 3 compatible now.  All files except `wb_download.py` should be python 3 compatible in the examples directory.  The file `wb_download.py` wasn't as clear how to go about fixing as the others because of changes in the structure of urllib and urllib2.  I think that is a minor thing that can be taken care of in the near future though.  Submitting PR.