Add tests for Clip class and improve its documentation

[mondeja]

• Add missing tests for Clip class.
• Improve Clip methods documentation.
• If start_time >= clip.duration in Clip.subclip method, raise ValueError (previously raised only if start_time > clip.duration).
• If end_time < 0 and clip.duration is None in Clip.subclip method, raise ValueError instead of printing a message to stdout.
• Raise ValueError instead of Exception in Clip.with_duration method.

• I have provided code that clearly demonstrates the bug and that only works correctly when applying this fix
• I have added suitable tests demonstrating a fixed bug or new/changed feature to the test suite in tests/
• I have properly documented new or changed features in the documentation or in the docstrings
• I have properly explained unusual or unexpected code in the comments around it

[coveralls]

Coverage increased (+0.8%) to 75.23% when pulling 2fd4190 on mondeja:Clip-tests into d213793 on Zulko:master.