Add extend() method (#17)

[ajcr]

Adds an extend() method to the rolling objects.

For example:

>>> seq = [1, 6, 3, 5, 2]
>>> roll = rolling.Max(seq, 3)
>>> list(roll)     # exhaust iterable
[6, 6, 5]

>>> seq.extend([4, 0])    # extend iterable with two new integers
>>> list(roll)            # two more max window values can be returned
[5, 4]