New tests

This adds new unittests and some fixes which I discovered with this tests.

The images are not included, so all tests will fail. I currently do not have a themend environment so all images would fail in such an environment. Please include the images (test_basic dir) from the result_images directory to ggplot/tests.

I haven't looked closely at the images, but some seem to be "wrong", so there are still problems in the code.