Adding an ImageFunction test which reads into an elemental variable
instead of a nodal variable.

The elemental variable field looks a lot more like the original image
when you have 1 pixel/element.

[original](https://drive.google.com/file/d/0B9BK7pg8se_iU2xFS19aUVVPOGs/edit?usp=sharing)
[elemental](https://drive.google.com/file/d/0B9BK7pg8se_iOUNSUlpfM1RNZUE/edit?usp=sharing)
[nodal](https://drive.google.com/file/d/0B9BK7pg8se_ieEw2S25VUU15S2M/edit?usp=sharing)

Refs #3544.
Merge pull request #3978 from jwpeterson/image_function_test

Adding an ImageFunction test which reads into an elemental variable