oc-mail: Use UTF-8 when they are parts with multiple charsets

If we have multiple parts with different encodings we recode
all HTML parts to UTF-8 and we use it as message charset.
This is neccesary because Outlook assummessa single charset
for all the message.

Also we convert the end of line in text/plain to <br /> tag
when showing them as HTML in multipart/mixed parts.