Add tests to mpl toolkits

This pull request adds infrastructure for testing of mpl_toolkits and a single test of axesgrid1 to demonstrate. The test is copied from a axesgrid1 example. If this is ok we should add more tests. 

Right now "python tests.py" runs all tests from mpl and the toolkits.
While matplotlib.test() only runs the matplotlib tests and mpl_toolkits.test() runs the tests of the toolkits only. 

So what do you think. Should tests be added to the toolkits? If yes any preferences to how the tests should be integrated into the mpl test suite.