Set up omniauth for twitter

- Used Dotenv for secret storing.
  - Assuming Heroku deploy we should use [heroku config:set](https://devcenter.heroku.com/articles/config-vars#setting-up-config-vars-for-a-deployed-application)
- Followed along with the [Railscast](http://railscasts.com/episodes/241-simple-omniauth-revised)
Create User model.

- Users can be created using a Twitter auth.
- Created a current_user helper method
- made the Conferences index the root_url
- creaed conditional display if user is signed in or out.
Add signout path.
Validates presence of User name, uid, and provider
User#find_or_create_using_omniauth returns nil with bad params
Add database cleaner
Test that User.find_or_create_using_omniauth finds or creates

- When the user exist the User count does not increase.
- When the user does not exist the User count does increase.
First SessionsController Spec
Sessions#create and #delete specs.
Enable Travis CI
Made rspec the default task for Travis CI
Configure Travis CI db
Merge pull request #3 from callbackwomen/add-twitter-authed-users

A user can be created and logged-in using Twitter Auth