Add support for doing multiple updates at once #1

Right now, each update is an atomic document modification. Updating multiple fields simultaneously requires doing a raw update with MongoDB operators. That's particularly painful for arrays with the $each required.

There should be some sugar like this:

$obj->update_multi(
    set => [ field, value ],
    set => [ field, value ],
    push => [ field, value],
    ...
);

The underlying calls should be split into an update statement generator and an executor. Then update_multi could just generate multiple operations and put them in one update document to fire off to update.

I think the "signature" of each should resemble the individual call. For example, if update_set only takes a pair, then set should take an arrayref with a pair. Later, if update_set is modified to take a hash ref or multiple pairs, then maybe it could be modified here.