Subtrees, manual activation, and better behavior!

Added

• Surveyor class to allow for regex-based URL matching (for manually setting 'current' URL).
• Climber::setCurrentUrl() method to manually set active URL after instantiation.
• Climber::isActivated() method to determine if any leaf is set active.
• Better testing for everything.
• Convenience functions (see README.md).
• "API"s for all classes in the form of interfaces. Hopefully this will improve backward-compatibility.
• Tree::subtree() method to create subtrees (sub-sections of existing trees).
• Forester class (extends Spotter) to feed Trees back into themselves (currently used when creating subtrees).

Changed

• Refactored system for setting active URL on Tree so that it is more accessible.
• Improved behavior of "activation" system.