Add tox for multi-interpreter testing
Makefile: Simplify the makefile and add Tox support
gitignore: Ignore the new .tox and virtualenv directories
maxcdn.__init__: Force absolute import so everything is consistent across interpreters
setup.py: Clean up the setup module and exclude tests
tests.int: Fail louder when environemnt variables have not been set
travis.yml: Use tox to run specific environments