Compatible with WinJS 4.0.0-preview

In addition to accepting strings, the style prop on react-winjs
components is now intelligent about its handling of numbers
and falsy values (similar to React's handling of the style prop).
  - Numbers. In general, "px" is appended to the number. For style
    properties which are unitless, the number is left as is.
  - Falsy values. Falsy values (except 0) are treated as the
    empty string thus clearing the inline style property.

- Fixed issue when BackButton causes the page to rerender
  7de4f0823829135a3d110dce77d98c2340ffb886
- Fixed #4: ListView broken when rendered inside of a
  Hub.Section