Add orderBy option to find method.

This patch adds orderBy option to find method. Now you can use find
method like below.

collection.find(query, { orderBy : { field1: db.ASC, field2: db.DESC } })

It will return sorted(appling field1 ASC first and appling field2 DESC
in the set that has the same field1) result.

Parsing orderBy option seems too difficult to read, it's because to
ensure backward compatibility.
As before, the second argument will be used as the limit number if it is
the number. And the second argument will be used as find option if it is
the JavaScript object.
If you want to use orderBy and limit option at the same time, this will
be as below.

collection.find(
    query,
    { orderBy: { feild1: db.ASC, field2: db.DESC }, limit: 10 }
)