Fix error in documentation of the suite runner function
Use the spec reporter by default in the suite runner

I can't think of a sane way to test this without introducing another untested
branch, printing garbage to stdout or resort to crazy things that don't really
constitute valuable tests :-(
Run tests in parallel by default

Why choose a slow thing by default? Running things without parallelism is
always slow with Overman, it makes no sense to have something bad as the
default.
Rename the suites parameter of the suite runner to files

It's clearer that way I think
Rename lib/reporter to lib/reporters

It will match better with how they should be exposed in the public API
Rename lib/interface to lib/interfaces

It will match better with how they should be exposed in the public API
Add module for exporting the public reporters
Export something in the public interface of overman
Use the public API in test.js
Export TestFailureError from the public API of overman

It's needed in order to tell if the suite runner failed because of an internal
error or because of a test error.
Don't use TestFailureError via private means from test.js

I want to make sure test.js uses only the public API, to ensure that the
API is actually usable from the outside
Show how to actually access the helper reporters from the outside
Rename value property of reporter error messages to stack

This makes it closer to normal Error objects and more convenient to
use.
Remove incorrect documentation
Make the suite runner print internal errors to stderr by default

This makes the suite runner easier to use in practice I think.
Merge pull request #51 from pereckerdal/public-api

Expose things in the public API