Make fishing success rate adjustable, and allow forcing a bite at any time. Adds BUKKIT-3837

This code adds two ways for plugins to enhance fishing.  It changes the constant values that represent fishing success rate to be variable and exposes them through the Fish entity.  It also adds a .bite() method to the Fish to directly force a bite on the next tick.

both features rely on https://github.com/Bukkit/CraftBukkit/pull/1045 to allow plugins to access the hook in the first place.

Leaky Ticket: https://bukkit.atlassian.net/browse/BUKKIT-3837