Check if ctools is installed before trying to check version, to prevent warning about version when enabling both ctools and node subpages at the same time

Beefed up Features support: when adding a node type to a Feature, add the subpages for it, and vice versa

Issue #1548500 by andirez, BrockBoland: Module will not install due to hook_requirements() dependency in .install file

Check for ctools in appropriate places. Upgrade Fields to use plugins (still need to update for Views)

Issue #1447992 by Argus, BrockBoland: Invalid argument supplied for foreach() in _node_subpages_get_field_value()

Issue #1447992 by Argus, BrockBoland: Invalid argument supplied for foreach() in _node_subpages_get_field_value()