Separate deleting a single metric from deleting a batch of metrics.

Here is my new attempt. Supports both batch and single deletes.

If you send a list of names you use this api call: http://dev.librato.com/v1/delete/metrics

If you send a single name you use this api call: http://dev.librato.com/v1/delete/metrics/:name

Some http libraries ignore the body of delete requests (example urlfetch: see how there is no payload for delete requets https://developers.google.com/appengine/docs/python/urlfetch/fetchfunction) 

This opens up the possibility of at least allowing single deletes to work.