Allow to "request" only a max count of items
Allow to "request" only a max count of items

This allow to handle items in steps by calling items(number) and afters
nextItems(number) until nextItems returns a result array with a length < number.
This avoids hanging the main thread of the webpage if a lot of items are in the
daap database.

bjoernricks/daap.js,bjoernricks/daap.js