featuresAt returns bucketType as designated in stylesheet, refs #876

Per https://github.com/mapbox/mapbox-gl-js/issues/876, this adds a `_bucketType` property to features returned in `map.featuresAt` that indicates the layer type specified in the stylesheet, so it would return something like
```JSON
{
    "_bucket" : "landcover_copy",
    "_bucketType" : "line",
    "_type" : "Polygon",
    "properties" : {
        "class" : "scrub"
    },
    "extent" : 4096,
    "type" : 3
}
```

This is a really simple fix for the specific problem I'm having in a project, but I think requires further discussion on the actual intent/use for `featuresAt` — is this :+1: or should it return more info? (Also, if this is sufficient, we should probably rename the returned `_type` property — `_geometryType`?)

cc @jfirebaugh @ansis @kkaefer @edenh @mourner @tmcw