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 
Huh? I'm not sure what conversion you mean, but if you're talking about storing whether the pot is pre- or post-1.7: knowing whether it is, between state creation and the invocation of this method, seems useless to me.