Initial version of testing framework

The testing framework is mostly a copy of the matplotlib framework
and uses most of the testing facility of matplotlib.

To write tests see the examples in  ggplot/tests/ or matplotlib/tests
and the matplotlib testing documentation:
http://matplotlib.org/devel/testing.html#testing
Add new tests to default_test_modules in ggplot.tests.__init__.py

To run the tests:
cd temp
python -c "import ggplot.tests;ggplot.tests.test()"
OR
python tests.py -v -d

Closes: #17