Include parallel-safe types from RcppParallel

[rorynolan]

I would love for RcppThread to have the parallel-safe wrappers that RcppParallel has. I prefer RcppThread because of the lack of need for TBB.

@kevinushey, is it OK with you to include this code in RcppThread even though it's MIT and RcppParallel is GPL?

If it is, I'll write some docs for RcppThread to mention the inclusion but I don't want to put too much work into this unless there's a green light from the people I'm taking the main code from.

Closes #73.

[kevinushey]

IANAL, but while this would be okay with me, I believe this would require sign-off from all authors on the RcppParallel repository.

[rorynolan]

Here goes: @romainfrancois @gregvdb @jjallaire

[romainfrancois]

I don't mind