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.
The invocation of GlowFlowerPot#update() calls TileEntity#updateInRange() if it's a post-1.7 pot, which in turn invokes TileEntity#update(GlowPlayer). This is indeed not overridden by TEFlowerPot, but neither is it in other TileEntity classes, except for TESign. Furthermore their isn't a method to send tile entity updates, so I thought it would fall out of the scope of this pull request to make one.