Support windows and macosx binary packages

[jankatins]

This commit adds support for windows and macosx binary packages.
Package type is automatically inferred from the filename.

It also creates the package structure
(./{src|bin/windows|bin/macosx}/contrib/{rversion/|}) if these
dirs do not exist.

Closes: #4

[jankatins]

from #13

A loose idea would be the following:
-- write a helper function or two which return book and answer isValidSourceFile() -- essentially by a grep against https://github.com/eddelbuettel/drat/blob/master/R/pruneRepo.R#L8 or variant thereof
-- add a similar helper isValidBinaryPackage() for binary packages (ie just foo_zip$ ?)
-- branch the code accordingly but very carefully

The current code is doing it slightly different: it asked what type the package is and constructs the path where to copy the package to accordingly. I could refactor both big switches into two functions which would do getPackageTypeFromPackage(inputFilename) and getFiledirForPackageType(packageType).

[jankatins]

I've updated the PR by moving the the main logic into (unexported) functions. I added the possibility to supply R version information to this functions, but didn't expose this in the main 'installPackages()` call.

The binary support is tested against the R repo for the jupyter R kernel packages, both for 3.1 and 3.2

[eddelbuettel]

Thumbs up for an actual Windows repository. I still have a Windows box at work (maybe not for long) that I should be able to test on. I still don't understand by you don't follow the standard naming scheme of using drat and gh-pages. People already find drat to be confusing, if we add another naming scheme more heads will explode. I do understand your point that it is "yet another" http-serviceable scheme from GH, but it is a) different leading to yet more explanation and) longer to type.

But in a way that's "your problem" and the README referenced above makes it clear -- I just shrug.

[eddelbuettel]

That said, I do like the new schme with the helper functions. That is much better.

[jankatins]

@eddelbuettel you mean why not add to a drat subdir? Mainly because I had to do it manually (script here) and forgot the subdir. And then never used drat to get packages from there, only install.packages(c("rzmq","repr","IRkernel","IRdisplay"), repos="http://irkernel.github.io/").

[eddelbuettel]

So supposed you add drat/ and moved the files / repo, then the usage would be

drat::addRepo("irkernel")                          # short, sweet, simple
install.packages(c("IRkernel","IRdisplay"))        # assuming the other two are Depends

which I find much simpler -- plus having the drat::addRepo() call in ~/.Rprofile means updates works automagically. Which may be the main point of drat. Anyway -- your problem :)

I'll play with Windows binaries tomorrow. I may have a use case.