Make all renderers have the same interfaces

The PDFChart and Chart renderers have been refactored so that they have
the same internal interfaces and public methods.

Closes #15
Fix getting the raw render on the pdf renderer
PEP8 changes
Add some unit tests and travis-ci.org job definition

Add run-tests.sh tool to run the tests. This is just an initial skeleton
of unit testing and there is a lot of work to do, but I think it is a
good base to start with.
Merge pull request #17 from alvarolopez/unit_tests

Add unit tests and fix code to get a clean build