third-party/build-deps.sh: Include third-party/README.md in archive
Additional refactoring of serializer interfaces

* Implement Features::HumanReadable && Features::Compact in
JsonCppSerializer::serializer

* Refactor serialize & unserialize methods; ::save and ::load are
provided for file I/O convenience. ::serialize && ::unserialize are
now only responsible for string (think: document) based I/O -- which
will see a lot of use in the unit tests that are coming up...

* Relocate Features enum from serializer interfaces to ISerializer
ptree/Value: Introduce find method & fix erase method

* Fix crash caused by checking for Value::array_valid() -- instead of
the object that we now use.

* Output the number of levels in depth traversed in ::dump method.
Additional refactoring of serialization interfaces

	JsonCppSerializer:
* Do not return false during serialization due to unknown types.
* Fix a few bugs recently introduced in JSON array & object
serialization.

	RapidXmlSerializer:
* Temporarily disable XML DOCTYPE output (was causing problems parsing
with nom::Value, since we don't handle that case yet).

* Introduce Feature::Compact feature

* Fix serialization of null values

* Fix issue with Value where if you had less than four (4) levels deep
worth of object nodes, serialization would fail to do so correctly,
leaving out a member (I think?).
Relocate ptree header files into its own file
SpriteSheet: Update code to latest changes in JSON interfacing

* Move SpriteSheet.cpp to JSON_SOURCE cmake variable.
Introduce unit testing files for Property Tree

* RapidXmlSerializer, JsonCppSerializer & Property Tree unit
tests (using GTest).

* Remove examples.cpp; our unit tests have replaced this file.

* Introduce several new json input files for future testing. (+1 squashed commit)
Squashed commits:
[53d51db] S