Add ability to filter by species model, with tests
Use separate SELECT statements for querying plots versus trees

The LEFT JOINs prevent using species.id in the WHERE clause, so a normal
JOIN needs to be used instead.
Fix filterStringToWhere tests
Update README.md
Merge pull request #7 from maurizi/topic/filterSpecies

Add ability to filter by species model, with tests
Add tests for filterStringToSelect.js
Add slightly more complicated test for filterStringToSelect.js
Add docco docs for JS files
Merge pull request #12 from maurizi/topic/filterSpecies

Improvements to tiler.js
Add 'make docs' command to make file