Add a `stop` command to the script.
Rename `run_ipcluster` to `ipcluster`.

Since it now has start and stop commands.
Add a module docstring.
Change the default number of engines to 4.

Only 4 are needed for our current tests.
Update the Makefile targets to match.
Wait until 'success' is printed.

Really, the longer string about engines starting successfully.
Remove 'sleep'--should no longer be necessary

... since the `start` command now waits for success.
Steal SciPy's gitignore.