Allow tight integration for Guard-Konacha

Guard-Konacha plugin is currently executing `rake konacha:run` when triggered. This is not efficient, and could have a major speed boost by having a single capybara session that navigates to the correct specs when triggered. To make this happen, konacha should allow tight integration. The following changes have been made to make this happen:

1. Be able to set 'mode=runner' when visiting the url. This will load the runner.js file in the view and allow guard to listen to konacha events.
2. Be able to provide an existing capybara session to the Konacha::Runner. For each test run a new runner can be instantiated using the same capybara session that stays active as long as Guard is active. (and speeds up test runs considerably)
3. Allow Guard-Konacha to provide a specific url to run instead of the hardcoded '/'

The pull request for Guard-Konacha will follow soon.