Added newrelic plugin

[mschae]

Hey Joseph,

thanks for this awesome bot. We have a lot of fun with it at BR.

I added a new relic plugin.

It can basically do two things: twke newrelic will list the most important NewRelic metrics. twke start|stop|check error watcher will watch the error rate and notify if it gets too high.

It's completely configurable via chat. The values are newrelic.account for account id, newrelic.application for application id, newrelic.api-key for the api key and newrelic.threshold for the threshold above which the error watcher should warn you.

Let me know if I can improve anything.

PS: This is slightly different from my first pull request. Error watcher now looks for error rate and response time.