Change Update method signature to make more sense

[iros]

Single row update:

ds.update({ _id : 4, prop : 23 });
// old method : ds.update(4, { prop : 23 });

Multi row update:

ds.update([{ _id : 4, prop : 23 }, { _id : 4, prop : 23 }]);

Flexible updates based on row:

ds.update( function (row) { 

  if (row.x == 12) {
    row.y = 144;
    return row;
  }

  return false;
});

Always need to return false for rows that dont' need updating so we don't run an update on all rows that haven't actually changed.

[iros]

Also resolves #163

[iros]

Also resolves #162