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
Merge pull request #9 from steventlamb/topic/custom_field_names

Add support for custom db column names
Merge branch 'master' of github.com:azavea/OTM2-tiler

Conflicts:
	config.json