Some internal changes.

* Engine is no longer dependant upon Derelict3, but instead uses DSFML.
* Removed Derelict3 SFML code.
* New third_party directory added to contain externals.
* Corrected some issues left behind by incomplete refactoring.
* D build script is now a suitable replacement for the PHP script. GNU,
LLVM, and Stupid compilers still unsupported, but support has been
extended to POSIX and OS X.