Add TODO persistence

* Add Datomic dependency and create temporary in-memory database per
  process.
* Simple schema for TODOs
* Ignore all but Datomic errors in logging
* CRUD for TODOs
Completed TODOs exercise
Add basic TODOs index page and shell create action
Finish todo#create
Add Bootstrap and redo index in Twitter Bootstrap
Make title attribute required
Add ability to delete TODOs
Add ability to toggle completion of TODO
Complete delete-all exercise