Add flower pot implementation

For this pull request is this pull request for Glowkit required: Glowkit#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.

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*Edited by turt2live*

**Related Links:**
- Glowkit: GlowstoneMC/Glowkit#48
- Ticket: #325 
If it is a pre-1.7 pot, both material data and block state will return the same. However, if it is a post-1.7 pot, only block state will return the correct data, since it isn't possible for the material data to know about the tile entity. So additionally, converting the pot to post-1.7 would make the material data of it utterly useless.