Add Blanket JS support for code coverage

To run it: `bundle exec rake jasmine:blanketjs`
Info on how to run tests, including coverage tests
Make code coverage env variable name more explicit
Merge branch 'js-code-coverage' of github.com:alphagov/limelight into js-code-coverage
Add jshint support and sample config
Add info on how to run jshint
Exclude vendor from jshint checks
Do not complain about jquery and require vars
Ignore underscore var for jshint
Allow use of '== null'
Merge pull request #195 from alphagov/js-code-coverage

Js code coverage and jshint