Add limit option to ScriptableCollection.find method.

Currently, ScriptableCollection.find method returns all matching results.
This patch makes the method limit the number of the results.

For example,
`collection.find(query)' will return all results, but
`collection.find(query, 3)' will return only the first three elements.

Actually limit doesn't limit the number of documents fetched from the network.
It just reduces the conversion overhead.