py.test support: Hide wait_for_match source code from backtrace

For failing assertions, py.test shows the entire source code of each
function in the backtrace. We don't want to see the stbt library's
source code for test failures (though we still want to see the details
for test *errors* --unexpected exceptions-- raised inside stbt).

You can override this at the `py.test` command line with `--fulltrace`.
http://pytest.org/latest/example/simple.html#writing-well-integrated-assertion-helpers
py.test support: Better output for `assert stbt.match(...)`

Instead of:

    def test_that_fails():
    >       assert stbt.match("template.png")
    E       assert <stbt.MatchResult object at 0x106db0450>

We get:

    def test_that_fails():
    >       assert stbt.match("template.png")
    E       assert MatchResult(timestamp=269493666, match=False,
                region=Region(x=180, y=0, width=...=160),
                first_pass_result=0.573794186115,
                frame=320x240x3, image="template.png")
make TAGS: Find python files in subdirectories