Added `containerStyle` property to `Grid`.
This property allows custom styling to be applied to the inner cell-containing div.
This can be used to enable padding within the grid.
For more context see: github.com/metabase/metabase/pull/3547

Refactored the `detect-element-resize` util to export a factory function.
This allows `AutoSizer` to import it initially but defer execution until mounted.
(Executing immediately causes problems for server-side rendering context.
Using a deferred `require` causes problems for es6 bundlers, eg Rollup.)

Fixed an edge-case that occurred for slower browsers when mounting and then quickly unmounting an `AutoSizer`.
In some cases, an animation event was left attached.
This event is now specifically checked for and removed.
Special thanks to @yb (PR #436) for this bugfix contribution.
