preliminary parallel support

For review...

I bumped the version to 0.9 only to distinguish my dev version (that we use in production) from the regular one.

As discussed, it especially works well on modern OSes (meaning supporting fork). Windows users can still use it by instantiating a cluster, and making sure it is properly initialized. 
I am sure there are a lot of stuff to tweak or improve.
 

Sorry, yes, that seems fine to me.  I'm planning on another testthat release in a month or two.