Added malt color calculations
Added toolbar to malt view controller
Prototype mostly complete: malt display settings view popup
Malt display settings view can be dismissed with 'done' button
Can now switch malt gauge between 'color' and 'gravity' metrics
Recipe KVO now includes notifying about recipe color changes

Also, we're now watching the batch size for changes. Callbacks to
to IBU, OG, and color keys will now get triggered on batch size
changes.
Can now choose which malt addition metric is displayed

Either "weight" or "% gravity"
Bugfix: Crash due to improper deregistration of KVO in OBRecipe

Symptom:
Set up about 5-10 recipes
Click a recipe, go back, click a different recipe, go back (repeat)
Eventually on the 4-6 recipe a crash will occur.

RCA:
We never unregistered observation on malt and hop objects when
the recipe was faulting.

Fix:
Unregister KVO that OBRecipe setup when it gets faulted or deleted.

Outstanding:
It is unclear whether or not malt or hop faulting would cause
these values to not get registered when they unfault. If this were
to happen, it seems like the UI would stop getting updated.
I have yet to force a malt to fault, so I haven't been able to
verify this.