lazy collection instance creation
complete tomdoc for API#item
move dynamic attribute definition to class method
add API::User class
add API#user method to retrieve a user
Implement UserCollection#entry and #entry_filter

This enables user retrieval via UserCollection#[] and API#user.
fix ItemCollection#where to handle merging tags conditions
ItemCollection: move #merge_ranges and #range_offset into new module
Implement UserCollection#where and document the whole class