Remove coverage generation from Shifter

As discussed in other issues, there's no need for Shifter to generate coverage files any more. This commit removes all traces of both yetitest, and istanbul. I've moved istanbul to being a devDependency as it's used for the coverage of Shifter itself.

Coverage generation should now be performed using other tools, for example:
* using Yeti; or
* a grunt task calling istanbul.

This will also remove the need for it to be documented as raised in #125