Added optional `db-handlers` parameter to defquery and defqueries

Optional `db-handlers` parameter added to allow use of other SQL
drivers. `db-handlers` is a map containing three keys:

* :insert-db-handler	a function used to handle inserts, defaults to yesql.types/insert-handler
* :execute-db-handler	a function used to handle commands, defaults to yesql.types/execute-handler
* :query-db-handler	a function used to handle queries, defaults to clojure.java.jdbc/query