Encoding and mailbox attrs

This is a "small" yet useful PR, with several features:

* Allow for complex, custom IMAP queries using new `custom_query` parameter
* Store mailbox attributes. This is better for i18n, since "All Mail" only exists on english GMail (and the only way to retrieve the "all mail" name is to look for "\All" in the attributes)
* Parse `from` and `to` headers (quoted printable content)
* Encode `body` and `text` from mail_encoding to unicode