Sequence improved api.

This pull request is completely based on https://github.com/dahlia/wand/pull/39 but improved and more simple api.

The original implementation had a strange behavior ever since modified the current image and never knew that was the image that really were using.

This is a concept for see if it fits better with the main library.

Sample code:

Open a pdf

    >>> from wand.image import Image
    >>> from wand.sequence import Sequence
    >>> i = Image(filename="../BBBBB.pdf")
    >>> i.has_sequence()
    True

Create sequence from image and access like a python list to sequence images.

    >>> s = Sequence(i, clone_by_default=False)
    >>> s[1].save(filename="/tmp/img.png")
    >>> i.destroy()

Checks if current jpg image has sequence.

    >>> i = Image(filename="tie01.jpg")
    >>> i.has_sequence()
    False

If we see that the result is as expected and convinces you, I will implement the corresponding tests.

Andrey