New example: contours without making an HDU

Based on http://stackoverflow.com/questions/21636395/how-could-i-convert-a-contour-plot-matplotlib-to-fits-format-with-header/21639693

![external_contour_example](https://f.cloud.github.com/assets/143715/2137201/b3f92048-9321-11e3-9761-044cebe8739f.png)