adds varnishncsa input

this pull request adds a varnishncsa input for simple logging event from Varnish cache.
more info on varnishncsa: 
https://www.varnish-cache.org/docs/3.0/reference/varnishncsa.html

here is an example of log message generated by the plugin:
```json
{
  "_index": "logstash-2014.06.05",
  "_type": "varnishncsa",
  "_id": "rf2Fz8LUQ66IvD5jA63sXA",
  "_score": null,
  "_source": {
    "@timestamp": "2014-06-05T11:03:53.000+00:00",
    "request_url": "/v12/accounts/105.json",
    "response_time_microseconds": "0.022854",
    "response_time_seconds": "0",
    "request_protocol": "HTTP/1.1",
    "response_size": "3713",
    "request_method": "GET",
    "query_string": "?api[bundle]=com.miteleliteandroid&api[bver]=2.0.30&api[device_model]=SM-G3815&api[os_type]=android&api[os_version]=17&api[sig]=E38DEA0EFC6C83C56CF37C9194BC6E15&api[timestamp]=1401966231&api[udid]=644410ad46d94f13&api[uuid]=533c8d59b",
    "message": "GET http://admin.applicaster.com/v12/accounts/105.json?api[bundle]=com.miteleliteandroid&api[bver]=2.0.30&api[device_model]=SM-G3815&api[os_type]=android&api[os_version]=17&api[sig]=E38DEA0EFC6C83C56CF37C9194BC6E15&api[timestamp]=1401966231&api[udid]=644410ad46d94f13&api[uuid]=533c8d59b HTTP/1.1",
    "status": "200",
    "http_user": "-",
    "request_header_x_forwarded_for": "31.4.181.86",
    "request_header_user_agent": "-",
    "request_header_accept_language": "-",
    "request_header_host": "admin.applicaster.com",
    "request_header_referer": "-",
    "response_header_cache_control": "max-age=0, private, must-revalidate",
    "response_header_content_encoding": "-",
    "response_header_content_type": "application/json; charset=utf-8",
    "response_header_last_modified": "-",
    "varnish_time_firstbyte": "0.022815943",
    "varnish_hitmiss": "miss",
    "varnish_handling": "pass",
    "@version": "1",
    "source_host": "ip-10-140-168-41",
    "type": "varnishncsa",
    "tags": [
      "applicaster2"
    ]
  }
```