Introduce IDrawable::RenderTarget typedef

* Alias for 'const Window&'
Add return statement when nom::Texture::draw errs
Convert GrayFrame && Gradient to std::unique_ptr

* Introduce IDrawable::UniquePtr, IDrawable::UniqueDrawables
* Introduce IFont, GrayFrame, Gradient pointer typedefs
* Introduce EventType typedef
Refactor nom::JSON from ground up

* Introduce 'sint' && 'uint' typedefs
* Introduce sprite_sheets example; cursors.json is now a shared resource

* Disabling the compile of json_reader & json_writer examples until I
get around to re-writing the function calls using nom::JSON API

  - nom::SpriteSheet
    * Refactored to use nom::JSON API
    * Disabled sprite width / height asserts for time being; calculation
      logic is faulty ...
    * Threw away JSON validation code in nom::SpriteSheet; it's way
      more trouble than it is worth! ;P
SublimeProject: show the Resources folder
Fix nom::JSON::Value::get_ints method

* Re-add Cursor::update method (debugging on TTcards)
Introduce nom::ticks function

* Wrapper for SDL_GetTicks