Implement validate_sql

[joshtemple]

Closes #6.

[joshtemple]

@tconbeer would appreciate you taking a look at this WIP. I'm not sure I've done it right. Doesn't the code need to return an error message or anything like that from the database?

[tconbeer]

No, you've got it right. It's kind of un-Pythonic but returning the empty string on error is expected. You'll know it's working if you highlight a valid query and the "Run Query" button says "Run Selection" instead. It should still show "Run Query" if the selected text isn't a valid query.