JsonSerializer: Initial clean up of code

* Introduce NOM_DEBUG_JSON_UNSERIALIZER_VALUES preprocessor define.

* Introduce unserialize_array, unserialize_object methods.
* Implement serialize_array, serialize_object methods.
Disable un-necessary builds
sanity.json: Add another mapped JSON object for testing
sanity.json: Add additional JSON types inside mapped obj for testing
Value: Re-introduce static member null
JsonSerializer: Additional refactoring -- reading deep hierarchies

* Introduce JsonSerializer::read_value method; we now support reading
unlimited depth of JSON objects within an array top-level node. See
last object in examples/Resources/json/sanity.json. This functionality
is only present within unserialization -- support for serialization is
coming up next!

* Temporarily diable unserializer test three.
ptree/Value: Fix debug output of null value types
ptree/Value: Disable Disable assert checks on data types
JsonSerializer: Additional refactoring -- reading/writing deep trees

Support for iterating through unlimited-depth JSON trees is now
complete.
CMakeLists: Re-enable building of examples & all library features
Revert "CMakeLists: Re-enable building of examples & all library features"

This reverts commit 4767c92b106e9ac7758c3d2c707729c1d0718ebe.
JsonSerializer: Break up serialization & unserialization methods

* The behind-the-scenes serialization/unserialization implementation
is now encapsulated by two new methods: write & read (respectively).
This means that the serialize & unserialize methods are now filestream
interfaces.

* The stringify method now outputs a JSON string of the input
nom::Value as expected.
ptree/Value: Fix copy assignment operator

* Each value type should now be properly copied over when using the
equality operator.
ptree/VString: Bit of clean up

* Introduce ArrayIndex typedef
* Comment out operator[] implementation; these are never used.
examples/values: Introduce yet another unit test
examples/values: Removal of old code
Re-enable building of all nomlib features
Initial refactoring of GUI
Additional GUI infrastructure

* Introduce GUI example app
* Introduce Button, ListBox GUI controls
* Introduce UIEvent & UIEventCallback support classes with experimental event
callbacks for Button & ListBox GUI controls.
Update codebase to reflect API changes from rebasing dev onto branch