Put GIF animation code into py script.

The animation script used in the KF chapter is general enough
to be used by the entire book, so I removed it from the notebook
and put it in gif_animate.py

code/gif_animate.py

@@ -0,0 +1,49 @@
+# -*- coding: utf-8 -*-
+"""
+Created on Sun Sep 14 12:41:24 2014
+
+@author: rlabbe
+"""
+
+from matplotlib import animation
+import matplotlib.pyplot as plt
+
+def animate(filename, func, frames, interval, figsize=(6.5, 6.5)):
+    """ Creates an animated GIF of a matplotlib.
+
+    Parameters
+    ----------
+    filename : string
+        name of the file. E.g 'foo.GIF' or '\home\monty\parrots\fjords.gif'
+
+    func : function
+       function that will be called once per frame. Must have signature of
+       def fun_name(frame_num)
+
+    frames : int
+       number of frames to animate. The current frame number will be passed
+       into func at each call.
+
+    interval : float
+       Milliseconds to pause on each frame in the animation. E.g. 500 for half
+       a second.
+
+    figsize : (float, float)  optional
+       size of the figure in inches. Defaults to 6.5" by 6.5"
+    """
+
+
+
+    def forward(frame):
+        # I don't know if it is a bug or what, but FuncAnimation calls twice
+        # with the first frame number. That breaks any animation that uses
+        # the frame number in computations
+        if forward.first:
+            forward.first = False
+            return
+        func(frame)
+
+    fig = plt.figure(figsize=figsize)
+    forward.first = True
+    anim = animation.FuncAnimation(fig, forward, frames=frames, interval=interval)
+    anim.save(filename, writer='imagemagick')