Length + ScaleFactor with statically-checked units
Implement Eq for ScaleFactor
Cleanup: Keep tests at the bottom of rect.rs
Implement mul, div, and cast for Rect
Add Length/ScaleFactor tests

Also adds missing `impl Zero` to make tests pass, and fixes a typo in a comment.
Minor copy-editing of doc comments
Replace deref() with an explicit get() method
Make the representation of ScaleFactor generic
Merge pull request #35 from mbrubeck/units

Length and ScaleFactor with statically-checked units