Made it possible to spawn several instances of charged items

This fixes a bug involving sinew spawning sinew*n, instead of n copies of sinew*10.
Rename itype-taking map::add_item() to map::spawn_item()

Per Kevin's suggestion. Hopefully this'll make it easier to find instances of that name later on.
Fix a potential bug with non-default quantity/charges

Whoops, forgot to update map::spawn_item() (formerly map::add_item()) to support non-default quantity AND charges. Now it should be possible to spawn N copies of an item with M charges.
Merge pull request #279 from ethankaminski/sinew-bugfix

Sinew drop rate bugfix, and ability to spawn copies of charged items