stbt.py: Run stbt template matching functions from an interactive shell

A context manager 'run' and a function 'run_cmd' that initialise 'Display',
execute the stbt functions specified within the context of 'run' or as
the argument of 'run_cmd', and tear down 'Display' in the end.

Their purpose is to let test developers execute stbt template matching
functions from an interactive Python shell; `stbt-run` doesn't allow using
these functions in test scripts.

Example:

>>> stbt.run_cmd(stbt.wait_for_match, 'example-image.png', timeout_secs=5)

and its equivalent with 'with' statement:

>>> with stbt.run():
>>>     stbt.wait_for_match('example-image.png', timeout_secs=5)

execute 'wait_for_match' in a Python shell, but

$ stbt run /dev/stdin <<-EOF
    import stbt
    stbt.run_cmd(stbt.wait_for_match, 'example-image.png', timeout_secs=5)
    EOF

raises UITestError.