[C+B] Add Entity.isOnGround() API. Fixes BUKKIT-3787

Addresses https://bukkit.atlassian.net/browse/BUKKIT-3787

Conversation with @amaranth regarding the event and its history http://gist.github.com/authorblues/430f5972071fbb1edbe7

This is a minor API addition to allow plugins to determine if an entity is on the ground. Currently, there is no suitable proxy for this information, since players do not have access to the onGround flag and the Block interface does not expose a suitable method for determining if a block can support the weight of a player. Therefore, rather than solving the latter problem (which is typically only used to solve the former), I propose a simple API addition. This is non-breaking, incredibly valuable to plugin developers, and solves a common and tedious problem that most developers are addressing with a tedious lookup table of blocks, which is not future proof.

In the event that a future change to the physics engine changes the way this flag is calculated, I purport that the notion of whether or not an entity is supported by a block is fundamental enough to Minecraft logic that such information will be made available there as well.

Bukkit PR: https://github.com/Bukkit/Bukkit/pull/796