mp4-dash.py should read from $PATH before requiring --exec-dir=

[nickdesaulniers]

➜  tmp git:(emscripten) ✗ echo $PATH
BunchOfStuff:/Users/Nicholas/mozilla/Bento4/builds
➜  tmp git:(emscripten) ✗ ls /Users/Nicholas/mozilla/Bento4/builds
CMakeCache.txt      libap4.a            mp4compact          mp4encrypt          mp4split
CMakeFiles          maybe_frag.mp4      mp4dcfpackager      mp4extract          mp4tag
Makefile            mp42aac             mp4decrypt          mp4fragment         tmp
aac2mp4             mp42hls             mp4dump             mp4info
cmake_install.cmake mp42ts              mp4edit             mp4mux
➜  tmp git:(emscripten) ✗ ls
video_00500.mp4  video_00875.mp4  video_01625.mp4
video_00500.mp4_ video_01250.mp4  video_02000.mp4
➜  tmp git:(emscripten) ✗ ../../Source/Python/utils/mp4-dash.py video_0*
Executable directory does not exist (/Users/Nicholas/mozilla/Bento4/Source/Python/utils/../bin), use --exec-dir