Pull filter traversal utility functions out of filterStringToWhere
Add support for filtering on tree photos

 - Moves JSON parsing from filterStringToWhere up into makeSql
 - Changes SQL generation to only JOIN to tables needed for filter
 - Gets rid of special-case base SQL for UTF Grid requests
 - Note: we do not need to do a DISTINCT ON (treemap_mapfeature.id), even
   though JOINing to photo means we may have more than one row per mapfeature,
   since we also do DISTINCT ON (the_geom_webwercator), which should eliminate
   duplicate mapfeatures
Update docs
Rename functions now that they use objects instead of strings
Merge pull request #34 from maurizi/support-treephoto

Support tree photo filtering