Add the concept of special events to the app

- Create special events model and migration
- Run migration
- Create special events controller, along with index and show views and modified routes
- Add pry-rails gem for troubleshooting
- Add Marathon 2015 event to seed file
- Remove static events view and events action from static controller
I left this view simple because I'm not sure what info will be relevant for each event. It requires rendering a custom partial for each event but, considering we only do a few of these each year, I'm all right with it. I'd rather have the flexibility to use images instead of just throwing some typical event info up.