Start running visual tests again

[springmeyer]

We temporarily lost the ability to run the visual tests on windows when the python bindings moved out: 4ccf161.

But Mapnik core now has C++ visual tests. Next actions are:

• Start building the C++ visual tests by adding them to mapnik.gyp
• Start checking out git submodule update --init the test data and visual test data
• Start calling the visual tests to be run along with the unit tests. Hope they don't have the same crashy problems as the unit tests: refs C++ unit test crashes on windows mapnik#2838