Make select_(nodes|ways|relations) return ints

This saves calling num_(nodes|ways|relations) if all you've done is the select_ call
Use return value from select_(node|way|relation) for single- and multi-object calls

This saves an index lookup against current_(nodes|ways|relations) for single-object calls and makes it easier for databases without history to handle missing objects
Merge pull request #27 from pnorman/node_visible

Refactor node visibility checks