stbt.py: Adds `is_black_screen` function and 3x tests

This adds a new function `is_black_screen` to detect the presence of a
black screen in a video frame. It converts the video frame to grayscale,
applies a binary threshold [1] with an optional mask, and then checks that
the maximum intensity value is zero. It returns `True` if it detects a
black screen and `False` otherwise.

Most of the logic was copied from code in the existing test
`test_using_frames_to_measure_black_screen` added in commit c96503e.
However, this version adds an optional image `mask` as well as making
the threshold value configurable.

This also adds three new tests for `stbt.is_black_screen()`:
* `test_is_black_screen_true_for_black_pattern`
* `test_is_black_screen_false_for_smpte_pattern`
* `test_is_black_screen_true_for_smpte_pattern_when_masked`

The third test above uses `tests/videotestsrc-mask-non-black.png`, a
newly added mask for videotestsrc that masks all the non-black pixels.

[1] OpenCV Tutorials: Basic Thresholding Operations
    http://docs.opencv.org/doc/tutorials/imgproc/threshold/threshold.html
test_using_frames_to_measure_black_screen: Use `stbt.is_black_screen()`

This removes the redundant function `is_black_screen()` from the Python
code in the test and replaces the the function call with
`stbt.is_black_screen()`. The latter function was added in the previous
commit.