This change corrects a few logging issues:

 * Standardized formatting errors with '%v'.
 * Standardized failure warning word usage.
 * Corrected an instance of using the standard log library when
   the gitea log library should be used instead.

Signed-off-by: Bo-Yi Wu <appleboy.tw@gmail.com>

* Implemented sendmail. This piggybacks on existing configuration to keep the change simple

* Changed privicy of new sendSMTP and sendSendmail functions

* Fixed Lint errors

* Seperated SMTP and sendmail into their own senders

* Making new structs private as they should not be used externally now

* Added sendmail setting to ini file

* Minor code cleanup

- Update import paths from github.com/go-gitea to code.gitea.io
- Fix import path for travis

See https://docs.travis-ci.com/user/languages/go#Go-Import-Path

* 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~

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.