Parallel on files and directories

A way to run mocha processes in parallel within an iteration.  The idea would be to take 2 new options:

`limit`
`paralellType`

This would make us use `async.mapLimit` to run a number of processes.  Say 1 for each file or 1 for each directory.

This PR is not functional at this moment, but I wanted to throw something up so that you can see what I'm thinking.

I would require this in the process file and replace the child_process stuff there...