Adding units to absolute scaling

Now asset generator accepts in, cm, mm and px as optional (px default)
suffixes to the width/height values. Converts as necessary using image
resolution.
Making changes for the pull request review.

- Unit strings are not limited to 4 units we have defined, analyze
makes sure correct units are used.
- Added more unit tests
- Adding DS_Store files to gitignore because otherwise they keep trying
to seep through cracks.
Changing units to unit for semantics.
Merge branch 'ba/abs-scaling-units' of https://github.com/adobe-photoshop/generator-assets into ba/abs-scaling-units

Conflicts:
	main.js
	test/test-parse-layer-name.js