Improve Vegeta as a library

This change set makes it possible to use the data that Vegeta used internally for it's reports, as a library user.
It breaks some APIs in order to do this (which I think is worth it at this stage), namely: `vegeta.Attack` doesn't accept a `Reporter` anymore. It simply returns a sorted (by timestamp) slice of `Result`s which everyone can use as they see fit.
The built-in reporters were converted to functions and do just this.

I have searched github for usage of Vegeta as a library and I couldn't find anything yet.
If any of the watchers of this repo is depending on the current API, please shout out if you think it should stay as is.