- Add RenderFractionalTranslation, a render box that does a
  translation based on a FractionalOffset.

- Make FractionalOffset more like Offset
  - dx/dy instead of x/y
  - add /, ~/, %
  - add .zero

- Add alongOffset and alongSize to FractionalOffset so that you can
  easily apply FractionalOffset to Offsets and Sizes. (Better name
  suggestions welcome.)

- Add transformHitTests boolean to RenderTransform (also on
  RenderFractionalTranslation), and to classes based on it.

- Remove the fade from Dismissable. We can add it back using the
  builder-with-child pattern like Draggable if we need it. See #1003
  for tha feature request.

- Rename a bunch of variables in dismissable.dart.

- Change the test for dismissable to not handle leftwards dismisses
  one pixel different from rightwards dismisses, and cleaned up the
  resulting effect on the test (mostly making sure we had the right
  number of pumps, with comments explaining what each one was).

Fixes #174.