Fix overriding keys with the same field

Previously the `preds` dictionary was only keyed on the field so when
DBH min came around the first time it would write:

     `{ tree.diameter:{'MIN': 5}}`

However, when DBH max came around it would just override it:

     `{ tree.diameter:{'MAX': 9}}`

Now additional predicates are appended to the inner dictionary.
Support an 'instance_id' parameter for all tile layers
Merge pull request #105 from ahinz/topic/update-js-with-instance-id

Add instance_id to tile requests