Add a new enchantement event on enchantItem

More information : https://github.com/MinecraftForge/FML/pull/446 and http://www.minecraftforge.net/forum/index.php/topic,18383.0.html
Could reduce the patch if you did the following:
```
ItemStack original = this.field_75168_e.func_70301_a(0);
ItemStack itemstack = itemstack.func_77946_l();
```
From what I can tell itemStack doesn't seem to be used anywhere else anyway.