* Do not write HTML in text/plain mail part

Fixes #2928

* Pass text/plain first, text/html second

* Do not send plain/text email if html2text failed (untested)

Also fix a possible race condition while install

- #1496: fallback plain text
- #1002: add date header
- #913: fix encoding of header

i've been missing a 'r' in previous, lol~

- fix delete user but repository watches are not decreased

New config keys in [mailer] section:
 - CERT_FILE: path to a certificate file.
 - KEY_FILE: path to a key file.

The User member of a message is not needed anymore.

The from that is send to the server, should always be the "system" from. This is also called the Bounce address http://en.wikipedia.org/wiki/Bounce_address

This lines got committed by accident. They do actually nothing, as SplitHostPort will give an error if port is not given.

The SendMail function is rewritten and has the following new functionality:
 - It is optional to skip verification of keys. The config option SKIP_VERIFY is added
 - If the port is 465, or ending on 465, the TLS(/SSL) connection is started first.