Fix typo
Test for items without captures

A uuid might belong to an item without any digital records. The
resources for these have no captures.
Implement #uuid method
Handle items with no captures

Some items have nothing in the digital collection. For these objects
the #items should return an empty array. Also add #has_captures?
method so say whether or not there are any items to retrieve.
Add SearchResults class
Unprotect #get_endpoint; Documentation

SearchResults instances need to call Client#get_endpoint in order to
get the next page of results, so this method can't be protected.

Also, add some documentation.
Add second page of results for 'birds' search
Implement #next method
Fix typo