v0.4.0: Complete test coverage

[stephane-caron]

This release brings documentation, full test coverage, and handles robot models installed from PyPI.

Also, it indulges in a project icon 😉

Added

• Coveralls for continuous coverage testing
• Document differential inverse kinematics and task targets
• Single-task test on task target translations mapped to IK output translations

Changed

• Argument to build_from_urdf functions is now the path to the URDF file
• Bumped status to beta
• Examples use the jvrc_description and upkie_description packages
• Use jvrc_description and upkie_description packages from PyPI
• Task is now an abstract base class

Fixed

• Unit tests for robot models