Replace `utf_8` and `latin_1` encoding names for prefered `utf-8`
land `iso-8859-1`.

Some email clients do not recognize utf_8 as the valid encoding, and according to the http://www.ietf.org/rfc/rfc3629, only valid name for the encoding is utf-8.
Merge hleumas-master branch.
Changelog for PR #21.