Add a "--version" option to the proxy.

[jodh-intel]

This will help users and developers to determine which version of the component is installed on a system; the CC 2.1 version or the CC 3.0.

See also: clearcontainers/proxy#69

[jodh-intel]

I think it would be worth displaying:

• a version string
• a commit id
• something to distinguish this version as being related to CC 2.1 (to make it clearer if the "wrong" proxy is being used).

[jodh-intel]

Do you think we could get this in for the next release?