[B+C] Added BrewEndEvent. Added getPotions method to Brew(End)Event. Adds BUKKIT-2441

#### The Issue:
There currently exists no way to retrieve the results from a Brewing Stand immediately as they are added to the three slots. There also doesn't exist a way to retrieve the three items as Potion objects without having to do so manually.

#### Justification for this PR:
Since no way exists to do this, plugins have to use workaround methods to retrieve the results exactly after they are complete, and thus these methods might not stay within the Bukkit API and abuse CraftBukkit and/or NMS. It is also frustrating to have to convert manually to a Potion object what should be provided by the event by default.

With BrewEndEvent, it has one extra method separate from BrewEvent, and that is the getIngredient method, which provides a cloned item of the current ingredient stack before it's amount is decreased. Using this it is possible to have ingredients with specific metadata to allow for a temporary method of adding new recipes to Bukkit. It also allows plugin creators to manipulate the potions (ie change name) or add functionality after brewing (ie shoot entities of the potions from the top of the brewer or something similar). The addition of getPotions() can aid in the comparison and manipulation of the resulting items.

#### PR Breakdown:
This PR adds another event class, BrewEndEvent, to be fired immediately after the resulting potions are brewed. It is fired prior to the removal of the ingredient to allow plugins to view a cloned version of the ItemStack of the ingredient.

#### Relevant PR(s)
BUKKIT-2441 - https://bukkit.atlassian.net/browse/BUKKIT-2441
CB-<> - <insert here> - Corresponding CraftBukkit Pull