Merge pull request #12 from rmarianski/update-column-names-query

update strategy for getting column names
default to an empty string for mapbox layer names
Merge pull request #13 from mapzen/empty-mapbox-responses

default to an empty string for mapbox layer names
default to empty string for oscimap too
Merge pull request #14 from mapzen/empty-oscimap-responses

default to empty string for oscimap too
remove redundant bbox intersection clause

Postgis performs a bounding box check as part of its ST_Intersects
implementation.
http://lists.osgeo.org/pipermail/postgis-users/2010-September/027675.html
suppress simplification at particular zooms levels
specify allowed geometry types per layer

Configuring the layers in advance prevents the query from having to
perform a check to ensure that the geometry type returned is consistent.