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