ietfparse: Create datastructures module.
ietfparse: Make headers a module instead of package.
setup.py: Implement a useful `clean` command.
changelog.rst: Mention datastructures module.
docs: Split api.rst into separate parsing and processing docs.
ietfparse: Split out errors into a module.
ietfparse.headers: Fix PEP8 violation.
setup.py: Move Janitor command into setupext.py
Add CONTRIBUTING.rst.