Implemented assignment and assignment-addition operators to Nepeta::Node. Improved read/write chains. Added STL style iterators.

Added a test program for some of the new features.
Added support for "casting" node arguments to new types, allowing reading/writing arbitrary types.
Fixed a few minor issues in parsing.
Added some more features to ReadChain/WriteChain. Primarily added reference-writing and storing node pointer support.
Tried to improve README, to make it easier to read.
Removed the old Nepeta::Iterator system, the new iterators replace them.
Tried to improve the Nepeta object's interface, to reduce calls to Nepeta::getRoot().
Fixed Nepeta::Node::buildLookupString() not making relative strings properly.