Check for convert and pngquant binaries (for merging in to issue-171 PR)

This augments PR #239 (which is the fix for #171).

At startup, it checks for the existence of both the convert binary and pngquant binary. If convert is missing, Generator can't function, and will shut down. If pngquant is not present, pngquant optimization is disabled and a warning is logged.

Also downgrades the photoshop-version requirement to 15.0.
This changes the interface of convert.savePixmap. This is technically a breaking change to our API. But, I'm not sure that the convert module is really part of our public API anyway.

@iwehrman what do you think? I don't think this warrants bumping our major version number. We could note it in the wiki though.