Add Fields[ResponseTime] to HttpInput

This allows for quality measurements from HttpInput messages. The
HttpInput plugin is now also compatible with the Circular Buffer Graph
Annotation (Alerts) plugin.
(http://hekad.readthedocs.org/en/latest/sandbox/graph_annotation.html)
Merge pull request #587 from christianvozar/features/http-input-responsetime

Add Fields[ResponseTime] to HttpInput