Allow headers to be passed to an HTTP Check

[alq666]

As requested by one of our users, here's how it can work:

instances:
  - name: my first service
    url: http://example.com
    timeout: 1
    headers:
      - content-type: application/weird
        user-agent: my-very-own
...

Once that bit is parsed, simply assemble the headers and send them along:

   def _check(self, instance):                                                                                                                              
        addr, username, password, timeout, include_content, headers = self._load_conf(instance)                                                                       
        content = ''                                                                                                                                         
        start = time.time()                                                                                                                                  
        try:                                                                                                                                                 
            self.log.debug("Connecting to %s" % addr)                                                                                                        
            h = Http(timeout=timeout, disable_ssl_certificate_validation=True)                                                                               
            if username is not None and password is not None:                                                                                                
                h.add_credentials(username, password)                                                                                                        
            resp, content = h.request(addr, "GET", headers=headers)   

[elijahandrews]

Thanks Tom!