Add detail screen

This actually isn't "finished", but I think it's worthwhile to get it
into master before working on the individual problems. I've created
issues for all of the known issues, but basically:

- [x] Properties from `TBEmployee` are implicitly unwrapped optionals. This
  can cause crashes if an employee doesn't have this info in their
  profile and we try to access it. Specifically, this is a problem for
  start dates and personal website urls.
- [ ] We're still showing cells even if the employee doesn't have
  information to use in that cell
- [x] We're being horribly inefficient with our use of `NSDateFormatter` in
  the employee view model. We should fix that.
Could this basic function of taking an optional string and returning either the string or an empty string be abstracted further?