run 2to3 to convert code to py3
py3 don't have Exception.message any more.

use args[0] instead
tune tests to make py2 and py3 both passes all

in py3, dict.items() have no exact order so I add a sorted to make sure
it gives consistent result
should support py3 now. add travis ci.
fix IPython capitalization
handle asynchronous or delayed loading of vega

in IPython notebook
update setup.py, add py3 classifiers.
Merge pull request #53 from minrk/IPython

IPython compatibility
Merge pull request #52 from lxyu/py3

Python3 support