Parse log file.

def parse(log_file):
    """Parse log file."""
    with io.open(os.path.expanduser(log_file), encoding="utf-8") as input_file:
        for line in input_file:
            if "Starting import of XUnit results" in line:
                obj = XUnitParser
                break
            elif "Starting import of test cases" in line:
                obj = TestcasesParser
                break
            elif "Starting import of requirements" in line:
                obj = RequirementsParser
                break
        else:
            raise Dump2PolarionException(
                "No valid data found in the log file '{}'".format(log_file)
            )

        return obj(input_file, log_file).parse()