Focus user button

### Overview
Implemented a "focus user" button.  If a list of users appears in the filter by user box, and this button is clicked, it causes this sequence of actions:

1. A new data query is issued, but with geobounds disabled - this retrieves all tweets associated to the selected users.

2. The tightest lat/long bounding box is computed, and the map zooms to fit that bounding box, automatically showing all of the user-associated tweets.

### Key changes

* Added a "focus user" button to the UI

* Attached a handler for the new button that causes the action sequence described above when clicked

* Changed ``retrieveData()`` to accept an options object, instead of the boolean flag it accepted before.  The flag (``saveUserList``) is not used anywhere in the application currently, but its semantics are included in the options object formulation

* Added a ``zoomToFit()`` method to the ``GMap`` class.  This consults the ``locationData`` property to figure out a bounding box and zoom the Google map object to those bounds.