Support jpeg for image transfer

[freeman-lab]

Sending images by writing to a file object now works in the python client, through the Lightning.image method, but it only works for format='png' (as the argument to imsave); using jpeg fails during the post request with a 500 error. Can others try this? Maybe an issue with my local image libraries, but maybe a problem on the backend. Fine to stick with pngs, but I thought jpeg might be faster to transfer.

[mathisonian]

@freeman-lab I got the following error when I tried using format='jpeg'

E           ValueError: Format "jpeg" is not supported.
E           Supported formats: emf, eps, pdf, png, ps, raw, rgba, svg, svgz.

it seems like imsave doesn't support jpeg. I'll keep improving the error handling on this so we can start getting more informative messages than 500 error