Add Select* helper interface methods to SqlExecutor

This PR adds `SelectInt`, `SelectNullInt`, `SelectStr`, and `SelectNullStr` methods to the `SqlExecutor` interface. This is useful when you want to write code that can take either a `DbMap` or `Transaction` and that uses one of these helper methods.

It's possible you left these out of the original interface pending another refactor, to avoid breaking API compat. In that case, we can certainly live without these changes; they are conveniences more than anything else (since we can always call `SelectNullStr(SqlExecutor, ...)`).

@ottob +1'd this: https://github.com/sqs/gorp/commit/c9c25a8f56cff5b9133231607136c62f8f9cd119#commitcomment-3485414