Mailhog

- updated mgo dependency
- temporarily remove vendor directory, because of some build issues
- Fix 117 - add info about proxies
- Fix 146 - keep tags on docker hub
- Fix 148 and mailhog/storage7 - leading \r\n in maildir storage
- 113 - better support RFC support for headers
- 118 - optimise Dockerfile for automated builds, thanks blueimp
- 122 - add -version flag, thanks swordbeta
- 123 - add .gitignore, thanks swordbeta
- 136 - update MailHog-UI assets, thanks sdurrheimer
- 145 - fix a link in DEPLOY.md, thanks nnmer
- mailhog/MailHog-UI22 and 104 - support TLS websockets in HTTPS mode, thanks blueimp
- mailhog/MailHog-UI23 and mailhog/MailHog-UI24 - support multiple instances of CID, thanks kieran and jerbob92
- more things I've forgotten/can't find!

- Extract SMTP protocol into isolated library
- Better protocol tests
- Add hooks for manipulating protocol behaviour
- Merge 14 - fix display of multipart messges, thanks derwassi
- Merge 17 - fix API v1 docs, thanks geerlingguy
- Fix 11 - add build documentation
- Fix 12 - support broken MAIL/RCPT syntax
- Fix 16 - add deployment documentation
- Fix 18 - better server-sent event support using [goose](https://github.com/ian-kent/goose)

- Fix 6 - Make SMTP verbs case-insensitive

- Fix 5 - Support leading tab in multiline headers

- Add 4 - UI support for RFC2047 encoded headers

* Configure from environment
* Include example Dockerfile
* Fix 1 - mismatched import path and repository name
* Fix 2 - possible panic with some MIME content
* Fix 3 - incorrect handling of RSET