CreateTableOpts(): create tables with additional options (ie "IF NOT EXISTS")

I couldn't find a way to create gorp tables with the "IF NOT EXISTS" modifier. 

* I renamed CreateTables() to CreateTablesOpt(), and added a bool argument to specify whether IF NOT EXISTS should be used.

* CreateTables() is now a wrapper around CreatTablesOpt(). Its behavior is unchanged.

I have only tested this on SQlite.