Template test case for graphic contexts

This is a PR that factors out a template test mixin to be used for testing the basic functionality of the kiva backends. This PR based on the work of peowebster:test/drawing-tests

pending:
- [x] add all the current kiva backends
- [x] update dependencies for travis-ci builds
- [x] fix errors
- [x] cleanup

The decorator can be attached on anything and it will cause the current test_method executed to be skipped. However, you are right, there are better places for the decorator