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