Add limit option to ScriptableCollection.find method.

Currently, ScriptableCollection.find method retruns all matched result.
This patch makes the method limit the number of the result.

For example,
`collection.find(query);' will return all result, but
`collection.find(query, 3);' will return only three elements of it.

Actually limit doesn't limit the number of document on network. It just
reduces the conversion overhead.