Ajaxified vote submissions

- Up/downvotes for prompts are now submitted via ajax
- Modified controller to handle html and ajax requests
- Error handling for ajax requests
- Turned duplicate vote check into custom validation method
- Having trouble getting vote feature tests to pass now,
  need to figure out how to get Capybara to test using
  javascript (selenium-webdriver gem added, no luck getting
  it fully working)
Merge pull request #39 from apesic/ajax

Ajaxified vote submissions