fix: support coturn use-auth-secret TURN auth mode, close #39

[rg0now]

Modify the longterm auth handler to implement the whole server-side authentication logic:

• split the username by the colon delimiter
• take the first item that looks like an integer (strconv.Atoi is successful)
• consider this item as a UNIX timestamp defining the expiry of the timestamp
• fail authentication if the the timestamp is in the past: timestamp < time.Now().Unix()
• otherwise take the original username, perform the HMAC and return the resultant hash
  Document the changes in doc/AUTH.md and add tests.

[coveralls]

Pull Request Test Coverage Report for Build 3586742408

• 13 of 13 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.2%) to 67.775%

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Totals
Change from base Build 3586212947:	0.2%
Covered Lines:	734
Relevant Lines:	1083

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💛 - Coveralls