Add tests for VideoClip and root __init__

[mondeja]

• Test __init__.py preview methods.
• Rewrite test for VideoClip.save_frame using BitmapClip and improve save_frame documentation.
• Add some tests for VideoClip.

• 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 (+0.2%) to 75.444% when pulling 4beabf5 on mondeja:init-videoclip-tests into 0bdc91c on Zulko:master.