[DO NOT MERGE] 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.
Cool, I think a good rule of thumb for bowndler is "include it anywhere that the bundle/bower binary us used". Will move it to the dev group.