Layout and interface refactor

(DO NOT MERGE)

This is a strawman proposal for how the loads.js runner can support higher load/fewer processes as discussed in https://github.com/mozilla-services/loads/issues/171

It does away with the mocha reporter, which is only capable of running a single hit per process.  The custom runner.js is changed to execute a full cycle based on either a fixed number of hits, or a fixed duration.  This means you can get a lot more load out of a single instance of this process.

@ametaireau thoughts on the approach at this stage?  I will attempt a matching refactor of the loads external runner and see how it looks.