Encoding and mailbox attrs

[Neamar]

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

Sorry, should have broken this into several smaller PR.

[Neamar]

EDIT: this has grown of its own to handle all special cases i kept seeing. This may not be good to merge now. The choice is yours.