New feature spec verifies error notification for empty ‘Selector’ input fields.

Spec verifies that an error message (actually, a dynamically-created `div` containing the message) is displayed when the ‘Select’ button is clicked when the selection-start and selection-end ‘Selector’ fields are empty.

In other news:

* Added ‘selenium-webdriver to Gemfile so that Capybara can run JS-dependent specs
* Page now sets focus to selection-start selector field when page is loaded.
* Finally figured out that the examples of the “new” Bootstrap 3 forms were not the examples I should have been looking at; form in index view is now testable (I want my day back!)