Added ability to select image format

A lot harder than was anticipated - partly because PIL doesn't write out
or preserve Exif data in any way, partly because there's no good (and
well packaged) Exif libraries for Python 3 currently, and partly because
Exiftool (the chosen stop-gap measure) has several annoying bugs...

Also made a start on hacking together some scale bar stuff for lens
selection
Merge branch 'master' of spider:~/picroscopy

Conflicts:
	picroscopy/templates/settings.pt