Handle raw strings in client test dumping

When given the following string in a test:

    r"""
    >>> a and \
    ...     b
    """

ok.py would translate this into

    """
    >>> a and \
    ...     b
    """

Since the dumped version is not marked as raw, the next time the test is loaded, the `\` will be treated as an escape character, instead of an actual backslash. This PR fixes #248 by adding a `r` marker to every multiline string. Single-line strings are converted simply by using `repr`, which escapes the backslash.