Allow non-strings to be logged without raising a TypeError

There are times you want to log something that is not strictly a string, e.g.

    from astropy import log
    log.info(42)

At present this raises a `TypeError: cannot concatenate 'str' and 'int' objects`

This minor pull request ensures that the string representation of any object can be logged, which brings the behaviour in line with Python's built-in logging module.