Add toString to GeoJSON models

[tobrun]

Add Object#toString implementation to com.mapbox.services.commons.geojson classes

Current output:

com.mapbox.services.commons.geojson.Feature@5cd82fe

Expected output:

{
  "type": "Feature",
  "geometry": {
    "type": "Point",
    "coordinates": [125.6, 10.1]
  },
  "properties": {
    "name": "Dinagat Islands"
  }
}

[cammace]

Work for adding all toString methods has been done in #450 but is held up since we'd like to do something a bit more automated (like using AutoValue). It would be nice to get that decision made soon so we can move forward. For GeoJson the toString method could simply make use of the toJson method which creates a string just like the above expected output.

@zugaldia how would you like to move forward on this?

[zugaldia]

@zugaldia how would you like to move forward on this?

The AutoValue approach from #491 (which I prefer) is unfortunately a breaking API change. How about we just merge #450 for now?

[cammace]

Resolved in 3.0, you can either use toJson or toString which will result in a json string or the typical java object information respectively.