Added player inventory, block placement

[zhuowei]

(also a /give command)

[grahamedgecombe]

Is it necessary to store an array of SlottedItems in the inventory? That kind of ends up duplicating the slot number and also makes it harder to move items around (with the Item class, you can pass the objects around without needing to worry about the slot).

The SlottedItem class was originally intended to be simply used for passing items to the codecs in the Message objects.

[zhuowei]

I could change it to use Item objects. Also, Glowstone now have a different inventory implementation, which may be better than my version.

[grahamedgecombe]

hmm I'll see if I can take a look at the Glowstone version at some point.