Creating a generic file reader interface

This is a WIP still, but the idea is to add drag and drop support for GIS files onto the map.  I created a reader for a reduced geoJSON specification that is attached to the `glOsm` example as a proof of concept.  I haven't fully fleshed out the base `fileReader` class.

I think I will change the interface a bit so that rather than providing a single reader type to the map, there will be a function that will try to open the file with every available reader until one succeeds.  Then, there will be a map option to enable or disable drag and drop.