Warn when section ordering is not according to spec

[jnothman]

in #121 we added documentation suggesting that docstring sections should be in specified order.

This should be augmented with a warning when the sections are out of order. This was the consensus at #68.

[rth]

Somewhat related section ordering is now enforced in numpydoc.validate checker.

[larsoner]

Indeed, I think we should close this and people should use the validation function to check things like this. It's going to be a lot more usable than trying to read warnings during a Sphinx build.