Fix crash when rapidly scrolling

If you scroll too fast, this can come back with `nil` data. If that
happens, `UIImage(data:)` will fail and return `nil` even though the
type says that shouldn't be possible. This is going to be the behavior
with Swift 1.1 anyway, so this is a bit of futureproofing as well.
Rename view models

We don't need a cell specific view model. It seems like it's more useful
to have a view model that represents a specific user. That way we can do
any transformations we need to do in one place. This essentially turns
the view model into a facade for use in displaying the model.

To reduce confusion, I'm also renaming EmployeesViewModel to
EmployeeListViewModel, which is actually a little more accurate about
what it's modeling.
Add detail screen

- Basic design of the detail screen
- Add properties to EmployeeViewModel to handle the additional info
  required for the detail screen