Cache binary in system tmp

[tbranyen]

We should cache the binary in a system tmp directory so that repeated installations are significantly faster.

[maxkorp]

http://nodejs.org/api/os.html#os_os_tmpdir
Although we may want to consider something like storing them relative to NPM's path somewhere, in case of use by nvm/nvmw or such, because then we have to worry about cross version caching. Either that or we have to keep track of it on its own.

I would add that after pulling from our cache (wherever it is), we should probably test load it, and if it fails, clean it out, so we don't get in some sort of loop of a bad build.

[tbranyen]

Opened an issue: mapbox/node-pre-gyp#149 as I think it makes more sense in that library than ours.

[tbranyen]

Closing, we'll re-visit if the feature ever gets added.