PingURL plugin added

Checks if a URL alive or dead and reports response time in miliseconds. Sample dashboard is such:

![pingurl](https://cloud.githubusercontent.com/assets/4558966/4603027/17bbcb18-5158-11e4-90bd-a3d952190def.png)

Additionally has option to change timeout value for the URL connection. Configuration is such:

    pingurl:
      name: 'any name you would see on Dashboard of newrelic.com'
      host: host-name-of-the-page.com
      port: 80
      path: /path/if-necessary/
      timeout: 3