Re-instate testing of coding standards.

An attempt to implement @ajdawson's idea from https://github.com/SciTools/iris/pull/1322#issuecomment-56684552.

NB. This is an alternative to #1325, so if this gets merged then #1325 should just be thrown away.
I feel like this might be a bit clearer on multiple lines (and without the duplicate `if`).