Create reusable searchEventStream.js
Support address search in "Add Tree" mode

* Move more cleanup logic into geocodeUi.js
* Use Bacon for geolocation
* Use more Bacon in addTreeMode.js to help with state cleanup
Implement "After I add this tree..." options

* Marker stays visible but must be moved for "new" and "copy" options
* Return plot id from add_plot_view() to support "edit" option
Add MockSession class to test module

Useful for running test requests that trigger session activity but the
test case is not interested in what is get or set in the session.
Merge pull request #322 from RickMohr/geocode-for-add-tree

Support address search in "Add Tree" mode (and add more Bacon)
Merge pull request #324 from RickMohr/add-tree-options

Implement "After I add this tree..." options
Add support for multiple/non-local tile hosts

This commit removes the requirement that tiles need to be served from
the same host as the application and allows specifying multiple tile
server hosts to allow browsers to request more tiles simultaneously.
Merge pull request #312 from jwalgran/topic/multiple-tile-hosts

Add support for multiple/non-local tile hosts