Added support for Go's repository retrieval.

 - Simply adds the releavant meta link in the project's show action.
 - Information about this meta tag can be found here:
   http://golang.org/cmd/go/#hdr-Remote_import_paths
Use project methods instead of attributes.

- This should work correctly in all cases now.
- Added a new project method that returns the web url for a project without
  the scheme prefix.
Use web_url for web_url_without_scheme.

- Previus version was still incorrect as it ignored cases where GitLab would
  be installed in a custom location.
- This assumes that if this method can be calle, web_url will return a valid
  URL.
Move Go specific meta tag after the atom feed code.
Change web_url_without scheme to web_url_without_protocol.
Added a simple test to web_url_without_protocol.
Merge pull request #5958 from brunoga/go-repository-fetch

 Added support for Go's repository retrieval.