Enable box-sizing changes at the component level

Inherit the box-sizing property from the html element, which allows
components to easily override the specified box-sizing such that their
desired behavior works as intended.

This allows Susy to not be concerned with third-party components'
box-sizing, without breaking the desired behaviors.

Working example: http://sassmeister.com/gist/ba636f049722573b3afc

Inspiration: Chris Coyier via
http://css-tricks.com/inheriting-box-sizing-probably-slightly-better-best-practice/
Set box-sizing within @at-root

This allows users to nest the global-box-sizing mixin without breaking
the generated css.
Update box-sizing tests

Update expected output to reflect changes
Add $inherit argument to global-box-sizing and border-box-sizing

- This argument is optional and defaults to false (the original behavior).
  Meaning that this change will not break existing sites as it must be
  opted into.
- When $inherit is true, the html element alone is set to the specified
  box-sizing, and all other elements inherit from this property. This
  allows box-sizing to be changed on a single element and have it change
  on all nested elements (unless otherwise specified). This feature
  lends itself well to components that may have different box-sizing behavior.
- The changelog and relevant documentation have been updated to reflect these
  changes.
Update version number and mark as unreleased
Merge pull request #366 from jdlehman/master

Enable box-sizing changes at the component level