Add grunt jasmine runnner for running Javascript specs

Add a `nodejs`/`phantomjs`-powered `jasmine` spec runner.

This builds on @bfirsh's work in 1cf309e45c6a72a21bfb8fdc0d0126de331cd474 from https://github.com/alphagov/govuk_frontend_toolkit/pull/74.

Node/npm/phantomjs are already present within the dev VM.