JSCBC-83: Fixed multi operation arithmetic operation.

Removed incorrect incrMulti and decrMulti (which don't actually
exist) and replaced them with the correct method, arithmeticMulti.

Change-Id: I1271485c5e3fabf772efe7374616f934ab83bf10
Reviewed-on: http://review.couchbase.org/30792
Reviewed-by: Matt Ingenthron <matt@couchbase.com>
Tested-by: Brett Lawson <brett19@gmail.com>