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.
 

I agree. We could just take the parallel stuff away (from parallel.R), but leave the "testthat_results" typed results and associated methods.
+ export (if not already done):
  * find_reporter
  * source_test_helpers
  * find_test_scripts
  * test_file
It is pretty straightforward, the problem is that I'm in vacation right now until october.
I've no problem if you want to do it by yourself. In the worst case I could merge your changed, then do a request afterward, as you see fit.