max-concurrency hook property support

[ivanpesin]

I have implemented support for hook concurrency limits (issue #148):

   {
     "id": "sleep",
     "execute-command": "/tmp/sleep-test.bsh",
     "command-working-directory": "/tmp",
     "include-command-output-in-response": true,
     "max-concurrency": 2
   }

When the limit is reached, webhook will return 429 Too Many Requests error and show error message:

[webhook] 2017/08/26 11:02:43 sleep got matched
[webhook] 2017/08/26 11:02:43 reached concurrency limit for: sleep (max=2)
[webhook] 2017/08/26 11:02:43 429 | 114.527µs | localhost:9000 | GET /hooks/sleep 

Let me know if you want it to be implemented differently.

[ivanpesin]

I've based it off master mistakingly, removed this PR and created a new against development -- #167