Add $setOnInsert command

The upsert feature in Mongo is incredibly useful, but it often results in a lot of redundant $set operations when used in conjunction with counters/increment.

This would be incredibly handy:

toSetOnInsert = { }
toSetOnInsert['name'] = 'test'

toInc = { }
toInc['foo'] = 1

vals= { }
vals['$setOnInsert'] = toSetOnInsert
vals['$inc'] = toInc

db.foo.update({_id: "foo"}, vals, true, false);