SDLApp: Disable OS X Spaces support
CONTRIBUTING.md: Add a couple links to existing reference docs

For the sake of future documentation!
third-party/README.md: Bump used SDL library version up
Introduce third-party/build-deps.sh helper script
Add event data for mouse button clicks field

This field was introduced in SDL2 v2.0.2.

* Remove window_id field from JoystickButtonEvent; this field is
confirmed (see mailing list) to be intentionally left out of SDL's
joysticks API.
Introduce new keyboard action trigger; repeating key press

This feature allows us to act upon a key press dependent upon the
state of key repeat; see SDL2 docs on KeyboardEvent's repeat field.
Introduce test for JSON mapped objects
CMakeLists: Temporarily disable unnecessary builds
Introduce test for JSON mapped objects
json/JsonCppValue: Bit of clean up on existing API

* Introduce methods: object_type( void ) && members( void )

* This class is now officially marked DEPRECATED; nom::JsonSerializer
and nom::Value are expected to phase this interface out in months time.

* Introduce temporary serialize & unserialize methods;
nom::JsonCppSerializer class is being removed entirely, and these two
methods are necessary to maintain TTcards build until it has migrated
away from JsonCppValue.
system/ptree/Value: Fix several output formatting issues in dump method

* Introduce experimental nom::Value::append method.

* Move nom::Value::dump_key && nom::Value::dump_value from public to
private; the intended access API is through nom::Value::dump or
nom::Value::operator <<.

* Removal of the assert checks in all getter methods.
JsonSerializer: Major refactoring -- new features, too!

* Introduce support for JSON mapped objects -- yay! All we have left
now is mostly clean up work in this class.

* Use JsonCpp interface directly within serialize & unserialize methods;
this allows us to remove the redundant JsonCppSerializer class entirely.

* Use JsonCpp's interface instead of JsonCppValue for internal handling
of JSON serialization/unserialization. (JsonCppValue is now officially
marked deprecated).

* Move const JSONCPP_INDENTION_LEVEL & Feature enum from
JsonCppSerializer class to JsonSerializer.

* Introduce dump interface, deriving from nom::Value's implementation.

* Re-inherit from IJsonSerializer. I plan on *hopefully* designing an
interface that allows for swapping this parser class out for another,
in case of the need arising down the road.
JsonCppSerializer: Removal of class

* JsonSerializer now carries the work this class did.
SpriteSheet: Fix bug that caused the meta fields to overwrite another
examples/sprite_sheets: Introduce tests for loading & saving sheets
examples/values: Introduce jsoncpp_test_one test & bit of clean up
JsonSerializer: Fix typo resulting in failed file save
Re-enable building of all examples & templates