move s-sql:sql-ize to cl-postgres:to-sql-string, make exec-prepared accept any sql-stringifyable parameters

darcs-hash:9c738a3c74bf38c54d830ef19a6b98f18940f440

make simple-date dependency for cl-postgres and s-sql optional, add some system-loading magic to make simple-date automatically hook into these libraries when it is loaded after them

darcs-hash:17cdd04d383594c983ed4289db8378df029737c1

darcs-hash:060981eb47e21ff8701a109da86356e33239a827