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.
Matcaffe expects a BGR image mean. I made the output RGB for ease of plotting in Matlab. If you want, I can make both outputs BGR by default and add a flag to write RGB. Sound good?