Adds support for relative url root

[markstanley-nps]

Closes #103

This replaces PR #130 to add support for relative_url_root

@brenogazzola hopefully this is easier to review.

You had commented that the below may need to account for the relative_url_root

mount app.assets.server => app.assets.config.prefix

We use rack config (config.ru) to run rails in the subdirectory as below, so we don't need the above changed

map '/subdirectory' do
  run Rails.application
end

We are currently using this change in our development pipeline, and it has been deployed to production. For production the static assets are served up by a separate server under the relative url.

[markstanley-nps]

The suggested changes were a great idea and I have made them.

I was concerned that having both compilers and compile classes may be confusing, but have seen this used elsewhere

[brenogazzola]

Thank you for your contribution!