Inital pass at searchpath impl
- added environment variable string expansion code in PathUtils.h (GetEnvMap() & EnvExpand())
- added FileExists() function to PathUtils.h
- added searchpath functionality into Config:: class (setSearchPath(path), getSearchPath(expand), findFile(filename)) this hasn't be plugged into anything yet.
- added ocio_core_tests.sh.in so we can pass in some env vars for the unit tests

http://github.com/malcolmhumphreys/OpenColorIO/commit/80bfd194bd7fd3d787e7a8795ad63be6050702c6

.malcolm