Add ncpus argument to install2.r

[csgillespie]

• Added ncpus argument to install2.r: -n and -ncpus
• Default: getOption("Ncpus", 1L)
• -n -1: max(1L, parallel::detectCores())
• Parallel package has been around since R 2.14
• Changed some rogue assignment operators from = to <-
• Updated changelog

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Closes #62

[eddelbuettel]

I made some comments over in #62 where we had the prior conversation.

[eddelbuettel]

Just wanted to once again say thanks! for that PR. I carried the idea over to update.r (which I run daily or near-daily on one or two machines) and the parallel run I now get after adding options(Ncpus=4) is rather nice. Well played, my friend!