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 
From what I can tell though, the pot is saved in the same format it was loaded in. This is false behaviour and should be corrected. 

If it's an old version then it should be converted upon being loaded. After loading, only the newest version will be used, including for saving.