Parallel ready

As discussed in my previous pull request, I removed the parallel code and made the code parallel-ready, i.e. it is now easy to implement the parallel testing 
in another package. 

I merged your master, fixed some issues (R CMD check was not working at all), fixed some tests, integrated the new "skip" feature into the "testthat_results", removed the parallel code
and exported the functions I needed.

I also contacted Simon Urbanek to see if he's interested by the little hacked I made to add progress support to clusterApplyLB.