Add Shaded Polygon to LineProfile Plugin Output

This addresses #865.  The output now contains a gray polygon with the same shape as the overlay region shown by the tool.  I also caught a bug when the line endpoints were floats (`draw.line` did not like them), so I converted the points to integers (see line 171).  I tested this with all four line orientations (hence the check on line 163).  I am using a 32 bit install of Anaconda 2.7 on Ubuntu 12.04.

![screenshot from 2014-02-02 17 38 06](https://f.cloud.github.com/assets/2096628/2061720/0ae78db8-8c65-11e3-998a-67a158b4df1d.png)

![screenshot from 2014-02-02 17 38 21](https://f.cloud.github.com/assets/2096628/2061721/0dff41b2-8c65-11e3-8cfc-62fa42669c20.png)