Add option to convert image mean to python as well as Matlab

Also rename `convert_image_mean_to_mat.py` -> `convert_image_mean.py`

If you prefer, I can split this into two commits so the file rename is explicit. Currently, git thinks I deleted one file and created the other (which means history will be broken). But the file is small, so who cares.
Can't you add the RGB vs BGR output from Matlab as an option? Is RGB the expected format for images in matcaffe?