Add Generator.prototype.getPhotoshopVersion; specify a required Photoshop version in package.json; fail on startup if the current Photoshop version does not satisfy the required version
check for convert and pngquant binaries, disable pngquant if not present
downgrade photoshop-version requirement to 15.0
remove private member _applicationPath from photoshop object
access ps version via reference
Merge pull request #242 from adobe-photoshop/jrb/check-for-binaries

Check for convert and pngquant binaries (for merging in to issue-171 PR)
Merge pull request #243 from adobe-photoshop/jrb/version-from-ref

access PS version via reference (for merging into PR for issue 171)
Merge pull request #239 from adobe-photoshop/iwehrman/issue-171

Specify a required Photoshop version