Add prop-types to support React 16 RC

[codeheroics]

Since React 16 removes React.PropTypes, react-leaflet-div-icon breaks.

This PR adds the prop-types package and allows react & react-dom 16 RC in peer-dependencies.

[codeheroics]

Just as I open this I realize the most important bit (React.PropTypes) is already in #13 from a pull request 3 months old.

@jgimbel do you plan on merging this or #13, or is react-leaflet-div-icon not maintained anymore?

[codeheroics]

Hi, thanks for the merge! Do you plan on tagging a new version & publishing it to npm soon?

[cinthia3301andrad]

Hello, will this merge work for typescript typing?

I'm using typescript and I get the following error:
Cannot find module 'react-leaflet-div-icon' or its corresponding type declarations.ts(2307)