Implemented parsing of CHANGELOG.rst for REP-0132

The formatting of the `Changelog` class, implemented here, will be done in the tools which use it, e.g. bloom will be responsible for formatting for debian changelogs.

This pull request implements the `catkin_pkg.changelog` module and a convenience command line script, `catkin_test_changelog <path/to/CHANGELOG.rst>`, for users to tes their `CHANGELOG.rst` and this libraries ability to parse it.