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.
Adding support for units (in, cm, mm, px) to absolute scaling
Fixed* and refactored** unit-related code, added tests for analyzeComponent, madetheunittestfilenamesactuallyreadable

* Bugs: cache the document's resolution, use rounding for the pixel values
** Define the valid units only once, add assertions.js for custom unit test assertions and use them (somehow seem to have gotten lost)
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
Merge pull request #44 from adobe-photoshop/ba/abs-scaling-units

Adding units to absolute scaling
Unit tests for analyzeComponent call