Add a grunt task to copy assets.

Originally grunt was only compiling Sass files and moving them to
`/public` meaning any user added javascripts or images had to be stored
in `/public`. These changes also include copying javascripts and images
from the `govuk_template` and `govuk_frontend_toolkit` modules to
`/public` making them available to the prototype.

This also adds a task to clean the `/public` and `/govuk_modules`
folders and removes them from the git repo as they contain compiled
code.
Remove `govuk_modules` from repo

As this contains compiled code, it shouldn’t be included in the repo.
Copy assets to public and consolidate govuk copy task
Add govuk_frontend_toolkit static asset path to server.js
Merge pull request #1 from rpowis/asset_grunt_tasks

Copy assets to public dir and consolidate govuk copy tasks
Merge pull request #30 from alextea/asset_grunt_tasks

Add a grunt task to copy assets.