Show stbt.MatchResult frame=None deprecation warning

The deprecation warning introduced in eb658d20 wasn't shown by the
existing warnings filter. I have added the word "stb-tester" into all
other deprecation warnings in the stbt module, and changed the warnings
filter to match.
stbt.ocr: Perform Optical Character Recognition using Tesseract

Tesseract is an open-source (Apache License 2.0) OCR engine.

Note that we use tesseract's command-line interface instead of an
existing Python wrapper, to avoid licensing restrictions
(python-tesseract and pyocr are GPL) and deployment difficulties (none
of the python wrappers are available in mainstream Linux distributions'
package repositories, so stb-tester users would have to use `pip` or a
similar Python packaging system). To use `stbt.ocr` you only need to
install Tesseract, which is available in Fedora ("tesseract"),
Debian/Ubuntu ("tesseract-ocr"), and macports ("tesseract") package
repositories.

TODO: Replace the `_Rect` class used by the image processing code with
the new `Region` class.

TODO: Add more-convenient ways of specifying a region. For example,
search for a label using `wait_for_match`, then specify the region
immediately to the right of that label (offsetting the region by the
width of the label template image).

Note that the unit test `test_ocr_on_static_images` violates our
standard 80-character line width, but only for the tabular test
data (which is more readable that way), not for the code. I think this
is an acceptable violation of the standard. Note that pylint only
applies this exemption inside of the array literal that contains the
"pylint: disable" directive.
stbt.ocr: Can specify Tesseract's page segmentation mode

If you have narrowed the region down to contain a single line of text,
you will often get better results with the `SINGLE_LINE` mode.

We default to Tesseract's default mode (fully automatic page
segmentation, but no Orientation or Script Detection).
stbt.Position: Tidy up docstring