ENH: Add a runtests.py script for running the test suite

The all-singing-all-dancing script builds Numpy and runs the test suite of the built version. This is useful for development, as you don't need to set up any sort of a dev environment for using it (in-place build, virtualenv, whatnot), you can run individual tests, tests only for a submodule, etc.

In principle, the `tools/test-installed-numpy.py` script could be dropped in favor of `runtests.py -n` but I didn't do that here as it's used in travis and I didn't want to go messing with that now.