Added "env" app.

This mimics /usr/bin/env, but because it is a gaffer app, it starts with an environment which includes $GAFFER_ROOT/bin in $PATH and $GAFFER_ROOT/lib in $LD_LIBRARY_PATH/$DYLD_LIBRARY_PATH. This means it can be used to run the utilities shipped with Gaffer (maketx, oiiotool etc) with a minimum of faff.