Events feature

Pivotal Tracker stories: [Add Events CRUD](https://www.pivotaltracker.com/story/show/66655876), [Add countdown to next AV Scrum](https://www.pivotaltracker.com/story/show/64299418)

* Created a new model for Event
* Added basic CRU methods to `events_controller`
* Implemented [IceCube](https://github.com/seejohnrun/ice_cube) gem for creation of recurrence rules and fast querying.
* Created basic UI for creating, updating and displaying events
* Added cukes and rspec tests (view, controller and model specs)