Foursquare venue query node and tests (issue #42)

This node can be merged as it enables the user to explore "food", "outdoor venues" and "sights" near the provided location (lat/lon coordinates). It returns the first recommended venue from the ordered list as determined by Foursquare. 

There are of course extensions which can be made (not limiting to 3 categories of venues, not limiting results returned to just the first recommended venue) but these can be done at a later time in other pull requests.

(Part of issue #42)
The conclusion reached in node-red/node-red#399 is that if a property is set in the node, it cannot be overridden by the message. We obviously didn't do a good job communicating that around the team.

I'll review the other web nodes to make sure this is followed elsewhere.