Updated to 1.9.1-SNAPSHOT for development towards the next version.
Let maven automatically insert the correct version in plugin.yml
Updated Metrics to version 6.
Reverted the attempt at automating the version number in plugin.yml.
Add a serializer for reading and writing ItemStacks to byte arrays.

To aid discoverability, we add a short-cut to this serializer in
PacketContainer, next to the getByteArrays() method.
This is a snapshot version.
Don't load classes on secondary threads. ADDRESSES TICKET-27.

We'll move the "load class" call to the main thread, hopefully fixing
ticket 27. The secondary thread will still call the "define class"
protected method, which may not be thread safe.
Add support for reading and writing NBT tags in packets.

This wraps the internal NBT read/write system in Minecraft.
Ensure that we can create compounds and lists.
Change "clone" to "deepClone", in line with PacketContainer.
Verify element type in tag list.
Added the ability to read NBT ItemStack tags.
Fine tuning of the NBT system. Added more test cases.
Added GPL license information.
Factory methods should return NBTWrapper.
Add the ability to serialize ItemStacks to strings.
Refactoring: Made NbtList and NbtCompound into generic interfaces.
Finalize the operation to make the NBT wrappers into interfaces.
Added the ability to serialize and deserialize NBT to many formats.
Inform about the purpose of the fromItemStack (fromItemTag) method.