update to new proj4js

Here's an update to utilize the [new proj4js library](https://github.com/proj4js/proj4js).  It's working (including tests) but I recommend holding off on the pull until everyone is ready since this will break compatibility with the old proj4.

Since both proj4 and Leaflet are now UMD-compliant, I updated `proj4leaflet.js` with an UMD wrapper styled after [jqueryPluginCommonjs](https://github.com/umdjs/umd/blob/master/jqueryPluginCommonjs.js).  I also updated the GeoJSON example to demonstrate the use of RequireJS for loading.  I could look into updating the other examples as well if there is interest.

Finally, I added a little code to convert URN-style CRS codes to the legacy style (e.g. EPSG:26915) since proj4 no longer does that internally (see proj4js/proj4js#49).