Add flower pot implementation

For this pull request is this pull request for Glowkit required: https://github.com/GlowstoneMC/Glowkit/pull/48. This basically adds all the necessary implementation for it.

**Modifications**
Add the implementation, as block state, for `org.bukkit.block.FlowerPot` introduced in the previously mentioned pull request. This is the only class that distinguishes the pre-1.7 way of handling flower pot contents (block data) and the post-1.7 way (tile entities). This implementation is also now registered to the item table. Apart from that a flower pot tile entity and block type implementation are added.

**Result**
Proper past, present and future proof handling of flower pots.
An implementation of the TE update packets and their use in a TE is in #267.

The reason why none of the other TE's override ```TileEntity#update(GlowPlayer)``` is because there is not outward changes upon NBT change. Whereas for items such as signs, banners, and pots, a change in NBT likely means a visible change - so that NBT should be sent to the player.