Properly convert null values to strings

Previously this would work for single null values, but failed for nulls
inside an IN caluse
Use the same join regardless of whether tree is in the filter or not.

Since we have added filters for tree.id IS NULL, we need to LEFT JOIN
when filtering for trees, which makes having a separate select pointless.

I've left filterStringToTables.js in place, since I expect to need
separate SELECT statements depending on the filter in an upcoming PR
Update docs
Merge pull request #33 from maurizi/support-display-filters

Support display filters