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

Now if your http library ignores the body of delete requests you
have the option of sending the indiviudal delete calls
use string_types to support python 2 and 3
Merge pull request #49 from Bachmann1234/separate-single-and-batch-delete

Separate deleting a single metric from deleting a batch of metrics.
Bump README year