Install in sub-directory support.

This changes Ghost core to allow hosting Ghost in a sub-directory. 

Additional work is needed:

  * Create new theme helper(s) to generate URLs for stylesheets/scripts (e.g. for https://github.com/TryGhost/Casper/blob/master/default.hbs#L17)
  * Change client-side JavaScript to properly send requests to `/<subdir>/api/*` and generate the admin menu with subdir.
  * Adjust tests to work with subdirectories (some work already done here)

The theme helpers require #1390.

refs #527