Remove Drinker class.

This was a premature optimization of the RegisteredPour class.

Also, make Keg an optional attribute of the Pour class.
Add Pour object to Flow.

Beginning of deconstruction of the complex Flow class.
Added the Pour object into the Flow class to begin
abstracting fields out of the Flow class into Pour.

- Delegate Drinker Name to the Pour object
- Delegate Shot to the Pour object
Refactor Images out of Flow class.

- Delegate images to the Pour object
- Also commit other changes for PourInProgressActivity which
  were missed in the most recent commits related to abstraction
  of the shout and drinker.