[WIP] drop old NumPy and SciPy support

As discussed on the ML, this PR drops support for NumPy < 1.6.1, SciPy < 0.9.

All tests except one pass in a virtualenv with NumPy 1.6.1, SciPy 0.13.1. The single failure is a `ValueError` thrown by SciPy, unrelated to our code.

I've not yet managed to get SciPy 0.9 to install on my machine, so I'd appreciate if others can try that.

There's also a bit more cleanup to do.

Ping @GaelVaroquaux, @ogrisel, @amueller.