Add support for building with Cmake 

building with cmake should give the following advantages:
- adds support for other OSes other than linux (though it's untested on anything but linux on my end)
- automatic discovery of arnold library and includes (based on `$ARNOLD_HOME` env var, or if the user has placed their arnold lib/includes on the standard library/include search paths).  the `FindArnold.cmake` comes from https://github.com/nerdvegas/vfxcmake
- set install location using `cmake -DCMAKE_INSTALL_PREFIX=/path/to/install`

If you disagree with any of the install locations, feel free to alter them.  I put the `.mtd` file alongside the library in the `plugins` directory because it needs to be on the `ARNOLD_PLUGIN_PATH` to be found. You could also consider adding that metadata to the c++ source file.

I also updated the README to explain how to build with cmake.  I changed it to github markdown while I was at it. You can see it rendered in html here:  https://github.com/LumaPictures/jf-nested-dielectric/tree/cmake

thanks for the cool shader!