Add IntegrityError check in case post title is already in database.
Was just running through your awesome tutorial and caught an error when playing with it. Figured I'd submit a PR.
 I appreciate the intentions, but your change-set has more problems than it attempts to solve. I've fixed the underlying issue with 6a07c8e1d949724ae10c9e304b1d7d30214cf2c6
 Oh shoot I didn't copy over my whole change. I had added an additional line after the flash to render_template('create.html') in my local code, forked and made a quick change in github's editor. But reading over your fix I suspect there's a good reason why that's also not the best option. 
 The reason it's not a good option (and whoops I need to fix something else, just remembered) is because you lose all your changes that you had been working on.
 Oh actually nvm, I see that the template is pulling straight from request.form. Nevermind!
