Updated the exporter and Engi renderer:

* Now supports export and use of material specularity.
* Updated the specifics of the rendering flags of materials
  with diffuse maps using texture that have an alpha channel.
* Added logic to the exporter to clear relevant files (png, jpg
  and json) from the chosen exporter directory so no old
  unreferenced files are left behind.
Updated the Blender exporter:

* Fixed bug in file cleanup logic.
* Fixed bug in logic used to determined whether to export a given
  texture as JPEG or PNG which led to textures not using their
  alpha channel to be exported as PNG rather then JPEG which made
  some exported scenes far too big. To give an impression of the
  quantitative difference, one test scene was reduced in size from
  7.7Mb to 3.3Mb total.
Minor change to the Renderer Material deserialisation logic.