Added convert_units() to cubes and coords.

This is the third instalment of the changing_units/convert_units work. See #284 and #320 for additional background.

This PR adds a ```convert_units``` method to cubes and coords.

~~It also improves the unit handling so multiplying and dividing a cube by a cube/coord with a convertible unit e.g K and celsius or Pa and hPa no longer requires user intervention to convert to a common unit.~~

It also fixes an issue with the ```iris.unit.Unit.convert``` method so it will now convert integers and arrays of integers.

(These other two features should really be in separate PRs, but they were developed during implementation and testing of #284.)

Reviewer: pp-mo