Real-World US-Customary Length Unit

New RealWorldUSLengthUnit model, which acts like a smart enum between US customary length unit point, pica, inch, foot, yard, and mile.

Includes a conversionRatioTo: method to find the multipliers/divisors to the convert between different RealWorldUSLengthUnits (with the same or different type values).  Includes Unit tests to make sure conversionRatioTo: behaves.

… And not much else other than that.  Primarily here to serve as identifiers for inclusion into other types (InchCoord) or to provide a bridge between others (e.g. pixels to/from inches).  Could end up serving as a good example of algorithms that act on little-to-no data.

Also, updated README.md to reference “feature/unit-models” instead of “feature/puck-dynamics”.
Update branch 'feature/unit-models' to latest 'feature/puck-dynamics'

Hand-merged:
	P-001.xcodeproj/project.pbxproj
Added license header to files missing it

In the same manner as before, added copyright and license entries to the top of each file existent on the 'feature/unit-models' branch but not on 'feature/puck-dynamics', and excluded files that already have the entries.
Minor: Removed unnecessary import

• Removed unnecessary import of CoreGraphics/CGGeometry.h (commented for use of CGPoint); I'm not sure how this snuck in.
Merge branch 'adhoc/travis-build-test' into feature/unit-models, forcing to the state of the former