Added devtools.sh target for monitoring unit tests

Added devtools.sh target for monitoring unit tests
Using `./devtools.sh wt`, it now does smart monitoring of unit tests,
and outputs a nice formatted test output, with failed tests at the bottom.

Relies on utility `watchfile`, which atm is not part of the repository.
    devtools.sh
Added noiseutils extension to libnoise

Added noiseutils extension to libnoise
    external/noise/include/noiseutils.h (+)
    external/noise/sdl2-cross-changes.md
    external/noise/src/noiseutils.cpp   (+)
    src/util/NoiseUtils.h               (+)
Added basic libnoise tests

Added basic libnoise tests
Just for testing that the library works.

    src/SConscript_tests
    src/util/TestNoise.h     (+)
    src/util/TestNoiseUtil.h (+)
Added automatic download of watchfile script

Added automatic download of watchfile script
This is extensively used by devtools. If it is missing,
it downloads the script from gists.github

    devtools.sh
    initialize_project.sh
Added PolyVox library entry header, added test

Added PolyVox library entry header, added test
    src/SConscript_tests
    src/model/PolyVox.h     (+)
    src/model/TestEmpty.h   (-)
    src/model/TestPolyVox.h (+)