Call sys.exit if one of the requirements is missing

Add simple test for conflicting github module (PyGithub3)
If required packages are missing, call Stop() in the main()

Allows the library to be imported even with missing modules with import scc.
Move HelperFormatter into parsers()

If argparse is not installed, then the class
definition of HelpFormatter will fail if it's
at the top-level.
Use non-zero Stop rc for missing modules

This makes it clear that a missing module
execution in no way succeeded.
Merge pull request #1 from joshmoore/github

Minor changes for missing modules
Merge pull request #49 from sbesson/github

Check for proper "github" module