Fix doc test failures after ScaleFactor changes

This package failed it's doc tests for me using:

    cargo 0.0.1-pre-nightly (603f29d 2014-11-01 19:29:48 +0000)
    rustc 0.13.0-nightly (ec28b4a6c 2014-11-04 03:36:55 +0000)

ScaleFactor now needs a number type annotation in addition to the units.
Incidentally needed to include use statements.

I'm just looking into Rust, so hopefully these changes are correct but comment if not.