Added support shift-left and shift-right operators

Added two tests as well – they pass. When installed locally, the following works:

    >>> import numpy, numexpr
    >>> a = numpy.array([1, 2, 3, 4, 5, 6, 7, 8, 9, 10])
    >>> b = numpy.ones((10, 10), dtype="int")
    >>> numpy.all(numexpr.evaluate('a >> b') == (a >> b))
    True
    >>> numpy.all(numexpr.evaluate('a << b') == (a << b))
    True