Example of embedding a figure into an existing Tk canvas

A new example that shows how to embed a matplotlib figure into an existing canvas at an arbitrary location. This is useful for integrating matplotlib output with graphical output that is already created using Tk drawing and not subject to be ported into matplotlib.

This example complements `embedding_in_tk.py` and `embedding_in_tk2.py` both of which use the FigureCanvasTkAgg function which creates a new Tk canvas.

![embedding_in_tk_canvas](https://cloud.githubusercontent.com/assets/1269002/4346755/71561724-4112-11e4-9ba9-b95684997d91.png)