Prepared statement support fixes + stmt naive merge

Hi all,

I've merged master into your stmt branch and fixed one or two bugs. I might have started on the wrong base branch (i.e. https://github.com/brianmario/mysql2/tree/stmt rather than https://github.com/sodabrew/mysql2/tree/stmt_rebase), but I think I have something working.

Fixes:
(1) spec/configuration.yml being ignored in some cases
(2) segfault when a string was passed into Statement#execute
(3) Used MYSQL_TYPE_DATE instead of MYSQL_TYPE_NEWDATE. To choose one, we'd need to do some sort of test at runtime.

Hope it's useful.