Use location of 0,0 in order to retrieve user instances when GPS is off

This is a stopgap measure.  The ideal solution is to create a new API
endpoint that does not use location at all, and only returns user
instances, preferably sorted by something other than distance.

Fixes #134
Merge pull request #135 from maurizi/fix/134/show-my-tree-maps-without-gps

Use location of 0,0 in order to retrieve user instances when GPS is off