Render base govuk_template, create development and production builds

TODO: govuk_template directory should be replaced with submodule
of compiled output of our underscore govuk_template.
Add a Gemfile for Sass, update the README
Delete govuk template files
Add underscore templates as govuk template submodule
Trying out new template with hard link and npm
Grunt and templates should work with mustache and node
correct path to base template
Add frontend toolkit as node and remove submodule
Update README
Convert head template to Mustache
Merge master into build-process-with-template-submodule
Fix stylesheet minification in production builds
Merge pull request #9 from alphagov/build-process-with-template-submodule

Build process with template submodule