Add dib-lint script

This will provide a place to put checks that catch common errors
in elements.  To start, this just checks that files starting with
a shebang are chmod +x so they can actually be run.

Change-Id: I4116a8f38f7bdfc5866764354c459fad8ca18e92

tox.ini

@@ -14,6 +14,7 @@ commands=
 [testenv:pep8]
 commands =
   flake8 {posargs}
+  dib-lint
 
 [testenv:venv]
 commands = {posargs}