Throw when attempting to convert empty filter to sql

The previous behavior of returning an empty string was causing downstream
problems because the sql being generated is actually a WHERE clause, and an
empty WHERE clause is invalid.
Merge pull request #6 from jwalgran/topic/fix-empty-query-handling

Throw when attempting to convert empty filter to sql
Add support for custom db column names

This is necessary because we specify nonstandard database column names for geometry types in order to work with mapnik.
Fix mislabeled function name in documentation
Add sftp-config.json to .gitignore

This is a config file for a SublimeText package.
Merge pull request #9 from steventlamb/topic/custom_field_names

Add support for custom db column names
Convert tabs to spaces in config.json

Fixing the result of a misconfigured editor.
Merge branch 'master' of github.com:azavea/OTM2-tiler

Conflicts:
	config.json
Add in boundary zone filter and additional predicate syntax

Previously the tiler support the simple concept of an infix binary
operator for SQL by making the following assumptions:

* The left hand side was always the column name
* The operator/matcher is the infix op and is appended to the LHS
* The right hand side was a function that converts a single value in the
  predicate to a single SQL literal for the RHS

When developing the boundary filter query to goal is to actually call a
function similar to:

ST_Contains(polygon, the_column_to_test)

Which doesn't fit the mold the matcher/value mold. The solution is to
break up the abstraction by providing a generic transformation object,
`predicateTransform`, in the predicates list.

That function can return a 'literal' value which is an underscore
template that will be evaluated with a single value, the column name to
render (called 'column')
Merge pull request #10 from azavea/topic/add-boundary-filter

Add in boundary zone filter and additional predicate syntax
Fix ref to renamed variable in server.js

In 58a8a0ad I renamed the existing config variable to windshaftConfig but
I did not update the reference to it in the console.log message.
Add support for WITHIN_RADIUS queries
Update documentation
Merge pull request #11 from steventlamb/topic/point_within

Topic/point within
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.