Add a utf8_grid endpoint to mock tiler

This enables the broken test to perform mouse click on the map
and achieve an infowindow bound to the plot id of the tree they entered.
make sure plotMover.onSaveBefore is always defined

Refactor the js so that this function is always defined. This fixed a javascript error that was occuring during the broken selenium test.
Fix plot marker to not err on (x, y)s with zeroes

The conversion from web mercator to lat lng was failing in the selenium tests due to the falsiness of 0.
Merge pull request #913 from steventlamb/bug/fix_selenium_tests

Bug/fix selenium tests