Add documentation for project services and display it

This PR adds support for project services documentation. It allows service owners to provide a short documentation that is displayed on the service page to help Gitlab user setting up the service.

It's done the following way:

* a markdown file must be placed in doc/services folder and its name must match Service#to_param value
* a new class called Gitlab::ServiceDoc is responsible for loading the documentations files and serves their content on demand
* a new initializer loads the documentation (read the files) only once when app is loaded
* a new instance method called Service#doc  provide access to this documentation from the Service instance
* the service's _form partial has been modified to display the documentation (using markdown)

Specs and spinach features are provided and I've added documentation for the Gemnasium service that I maintain