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.
Note this does **not** change the interface of `Generator.prototype.savePixmap`. It's unfortunate that these two functions are named the same thing. (The entire convert module is kind of unfortunate...)