Generate loan summary on home page

**Summary of changes**

1.  Realtime calculation of loan summary details when all fields are filled in (date validation check on loan start date field)
2.  Prepared to integrate loan details -- monthly/yearly amortization table -- will be added later maybe through a modal (showing this by default clutters the homepage)
3.  Added a spec with javascript enabled to test loan summary calculations (I added the `poltergeist` gem and loaded this in Rspec as the javascript driver)