Cache Traveling Items

There existed a race condition where either the client could pass an
item from one pipe to another (with intact ItemStack NBT data) or a
packet could arrive from the server to inject the item into the pipe
(without NBT data). This resulted in incorrect Icons being occasionally
displayed for a moment for Items that use NBT to determine the Icon
(Forestry Bees/Trees).

To eliminate this race condition, all Traveling Items are now cached
(weakly) and ItemStacks are no longer sent as part of the Traveler
Packet. The client must instead request the ItemStack from the server.
The bonus being that the ItemStack (NBT and all) needs only be requested
when it first enters the network.

This also has the affect of further reducing bandwidth usage by Item
pipes, though it should be noted that Item pipes where already the least
bandwidth intensive pipe.

Additionally I rewrote the Item Grouping code. I'd never looked at it
before, but I figured I should make sure it properly maintains the pipe
state after grouping. What I found was 200 lines of wtf code that was
horribly outdated and broken. I replaced it with 20 lines of code that
are less dependent on implementation details of how pipes work. -.-