Initial release

[rgwozdz]

This PR contains all the new code for the provider. The readme describes the usage and configuration requirements. Some general things to bring to your attention for feedback:

• I'm using the :host parameter to point to a key in the configuration object. The value of that key holds a serialization object which the provider passes on to S3 so it knows how to convert the file to stringifed JSON. The goal here was to allow a single S3 provider to access files that may have been serialized in various ways.
• The :id parameter holds the path to the S3 file (not including the bucket). Any forward slashes need to be replaced with :: (should we include the bucket here instead of setting it in configuration?).
• I use schema-matching to determine how to translate the S3 JSON to GeoJSON. Specifically, I check to see if the JSON returned is already a feature collection, a feature array (i.e., has properties and geometry), an array of geometries, or just a simple object array. Then I translate to a GeoJSON feature collection appropriately.

[rgwozdz]

@haoliangyu @dmfenton - the model.js and s3.js files have changed quite a bit since you first reviewed. Would you like to take another pass?