Add support for websockets when proxy is enabled

[dajudge]

Please ensure that your pull request fulfills these requirements:

• [X] The pull request is being made against the master branch
• [ ] Tests for the changes have been added (for bug fixes / features)

What is the purpose of this pull request? (bug fix, enhancement, new feature,...)

Fixes #142

What changes did you make?

Based on #528:

• removed changes unrelated to websockets
• rebased to current master.

Provide some example code that this change will affect, if applicable:

Handles HTTP upgrades used for websockets by forwarding them in proxy mode.

[dajudge]

maybe @thornjad can have a look at this? ;-)

[Ni55aN]

@thornjad