Publicize vegeta.Result
Decouple Reporters from Attack function

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.
Simplify Reporter objects to be simple functions
Use actual body length for BytesIn
Go lint warning fixes
uint16 for HTTP Status codes
Merge pull request #14 from tsenart/improve-library

Improve Vegeta as a library