Remove use of deprecated Horde_Mime methods
Un-deprecate quoted printable encoding, but move to separate class

Built-in quoted_printable_encode() method does not allow for specifying
EOL or line-length.  So keep a utility method around that wraps around
the PHP Q-P stream filter that does handle these paramters.
Use Horde_Mime_QuotedPrintable
There is no requirement that the value be a string.

The empty() call fails on strings.