Parallel Suites

This feature allows you to specify that a suite's child suites will be run in parallel.  Here is a code example:

```javascript
describe("parallel suite parent", function () {
  this.parallel(true);

  it("this test will be called in series before the child suites");

  describe("parallel suite 1", function () {
    it("will be run in parallel to suite 2", function(done) {
      someAsyncFunc("foo", function (err, res) {
        //some assertion
        done();
      });
    });
  });

  describe("parallel suite 2", function () {
    it("will be run in parallel to suite 1", function(done) {
      someAsyncFunc2("bar", function (err, res) {
        //some assertion
        done();
      });
    });
  });

  after(function () {
    // this will be called after the child suites complete
  });
})
```

I realize that this might be a bit out of scope for mocha, as it goes against some of the design decisions made in the past.  But, nonetheless I had a use case for parallelism, I didn't want to abandon the other features of mocha, and there is some demand for parallelism from others, so I patched it in.  This is related to issues #3, #19, #81, #98, #235, #309

Doing it at the Suite level seemed to be the most logical way to do it.  Tests within a suite are still run in series. Here's a high level overview of the changes:

- Change `Runner`s to wrap only a single suite, create child `Runner`s for child `Suite`s, inheriting the proper properties along the way (globals, bailing, etc....)
- Add a `parallel` property to `Suite`
- If the current `Runner`'s `Suite` is parallel, run child `Runner`s in parallel, otherwise run in series
- Have the parent `Runner` proxy events from child `Runner`s, re-emit them immediately if running in series, or batching them if in parallel, replaying the sequence at the end.  This prevents jumbled and interleaved reporting
- Add a `--parallel` CLI flag to run all root-level suites in parallel, if you want.

This also encompasses my previous pull request, #836.