Remove mas-development_dependencies and replace with equivalent code.

Today I decided to test out what `mas-development_dependencies` what actually doing by removing it and then adding stuff back in until the tests passed. This is what I came up with. Here's a brief summary:

* Requires a bunch of test gems.
* Sets up a base configuration for RSpec and Cucumber.
* Sets up rake tasks to run Karma.

The commit here is mostly copy-and-paste to add the above back in. Taking this approach generally would involve copy-pasting these files around and we all know how that ends up — very slightly differing versions of the same files across several different projects, and a corresponding itch to DRY up the lot.

Moreover, centralising agreed test config and non-trivial tasks like setting up Karma within a Rails env decreases the barrier-to-entry for testing, and that's always a good thing. It also standardises the test environment, which makes code reuse across projects easier.

There would seem to be an argument for adding some of this back into mas-devdep. It'd be great to hear your perspective on this, so we can all be on the same page when it comes to things like this in future, and decrease the likelihood of unexpected breaking changes.

CC @stevenwilkin @benlovell @baphled 
This would only affect the Mortgage Calculator repo in this context but we can't remove this line from `mas-development-dependencies`.