Added password recovery; made some other minor edits

Added password recovery and made some minor edits. These minor edits are: 
- Surrounded a `YES` by quotes. When this was not yet done, there was a PHP notice which made sure that the return data wasn't equal to `success`, but to `<a notice>success`, so the page didn't reload automatically.
- When sending an AJAX request, trim the return data before comparing. If it wouldn't be trimmed, and if you accidentially added trailing whitespace to the file, the comparison would always return false.
- When creating an account, check whether the username or email address are not yet added, not just username.