Provide more modulization

[miri64]

Towards providing different front-ends (e.g. a web service) some steps need to be taken:

• redirect stdout and stderr of both the script as well as called shell commands within the script (mostly git, maybe gnupg also generates some output...) to a logger
  • optionally: move to pygit2 instead of raw git commands, but this can be more tedious
• extend config with current constants
• Make functions not required publicly private ;-)
• move main() to its own module (riot-release-manager.cli?)
• Documentation, documentation, documentation