Added u() and b() compatibility functions.

In order to be compatible with Python 2 and 3, two functions have been
added.  A unicode string can be constructed as:

    from .compat import u
    u('string here just as with u"..."')

Also, a bytes string can be constructed as:

    from .compat import b
    b('string here just as with b"..."')

While the bytes string isn't needed (b"..." works in Python 2 and 3), it
is provded for consistency.