Implements a new format for the text reporter with more information
than before.
Also Rename Result.Timing to Result.Latency and related names.
Adapt README.md examples to account new text reporter format and
library changes.
This changeset breaks the API of Attack in order to decouple Reporters
and the Attack function. Attack now returns a slice with
non-deterministic order of Results which one can use on the calling code
with or without Reporters, hence making it much more useful on a library
usage setting.
These developments could be of interest to issue #11 which was closed in
the past.
