Use a local sdutil.
Support SD_ARGS environment variable.

Signed-off-by: Michael Elsdörfer <michael@elsdoerfer.com>
Merge pull request #7 from miracle2k/master

Support SD_ARGS environment variable.
Support buildpacks creating an empty release file.
Merge branch 'sdutil' of github.com:miracle2k/slugrunner into sdutil
Merge remote-tracking branch 'origin' into sdutil