fixup? Region: Add convenience methods

This was causing the following error from pydoc (invoked from
api-doc.sh, invoked from `make doc`):

    STBT_CONFIG_FILE=stbt.conf ./api-doc.sh README.rst
    Traceback (most recent call last):
      File "/opt/local/bin/pydoc", line 5, in <module>
        pydoc.cli()
      File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/pydoc.py", line 2328, in cli
        help.help(arg)
      File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/pydoc.py", line 1793, in help
        elif request: doc(request, 'Help on %s:')
      File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/pydoc.py", line 1532, in doc
        pager(render_doc(thing, title, forceload))
      File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/pydoc.py", line 1337, in pager
        pager(text)
      File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/pydoc.py", line 1435, in plainpager
        sys.stdout.write(plain(text))
    UnicodeEncodeError: 'ascii' codec can't encode characters in position 416-423: ordinal not in range(128)

The result was that the documentation for `Region` was missing entirely
from README.rst.

Does this change work on your system?
fixup! OCR: Add stbt.match_text()

This fixes the formatting of the code block in the docstring, and fixes
the following warning during `make doc`:

    <stdin>:463: (ERROR/3) Unexpected indentation.

The `::` introduces a code block. The `::` itself doesn't appear in the
generated html/man document.
fixup! OCR: Add stbt.match_text()