Added Frag component

Frag is a helper component for creating reusable React GoogleMap components. This
component has no functionality or output, it just allows you to return multiple
components from a custom components `render` function.

The only prop required is the `map` prop (the GoogleMap instance), this is
automatically passed down to all direct children of the `Map` component but
will need to be manually set if you use `Frag` inside a custom component. The
`Frag` component will then pass the `map` prop down to all of its direct children.
Merge pull request #8 from pieterv/frag-component

Added Frag component