A simple Makefile task for PEP8/PyFlakes checking

This is for #255. There's now a simple `make style` task that runs a PyFlakes and PEP8 check on all files, with some selected PEP8 errors ignored. The exact errors to ignore probably have to be refined, but it's as easy as changing the `PEP8IGNORE` variable in the Makefile. There's a couple lines in `developing.rst` about using this, under the Coding Standards section.