Mount PensionsCalculator (behind a feature flag)

* introduces bowndler for bower/bundler integration
* references pensions_calculator branch on github until pensions_calculator released gem is ready to be mounted

This PR should only be merged once the pensions_calculator is hooked in from a built gem.
I don't think this will work with the new nesting approach. I think it needs to change to:
```erb
<%= render layout: 'layouts/contrained' do %>
  <%= yield :engine_content %>
<% end %>
```