Alterations and additions made to main block of code.

* Fixed the Makefile to build each version independantly. Added a rule to build all at once.

* Renderthread version works but there are some huge problems with the code. It can cause deadlock on no work left.

* A new scheduler implementation can be attatched to the mandelbrot algorithm providing it has ws_start_threads(), ws_initialise_threads(),
  and the constant WORKER_COUNT is defined. A rule in the make file needs to be added to allow building.

* A simple sequential scheduler has been added.