Added include node and functions for new mapping methodology. For #37
Created stack of table-specific aliases in mapper. For #37
Fixed bugs in calling from() on a model. For #37
Fixed bugs in new mapping code. For #37
Fixed bug in old mapper keeping includes from working. For #37
Improved visit() calls to pass all arguments. For #37

Also removed some unneccessary private helpers that were being used to
build WHERE, GROUP BY, HAVING, and ORDER BY clauses.
Added getMap() method to relation for generating mappings. For #37