Add flower pot implementation

For this pull request is (this)[https://github.com/GlowstoneMC/Glowkit/pull/48] pull request for Glowkit required. 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 present and future proof handling of flower pots.