WIP - jshint integration

* Integrated with the asset pipeline.  If any linting errors are reported, it raises an exception halting the page load / asset precompilation.
* A rake task for a comprehensive linting report (``rake jshint``)
* Only lints files found within app/assets, which are assumed not to be minified.
* Pulls in the jshint config from the .jshintrc in the root of the project.

When the asset pipeline breaks we don't try and provide any in depth information because a) Rails' error reporting mechanism isn't well suited to reporting multiple errors which is often the case, and b) given the way that the asset pipeline performs aggregation of dependencies, it's not easy to programatically determine the exact location of the line that is raising the error.

The current report raises quite a few errors so those will need addressing before we merge this into master.