Cache x64 version of node.exe. Refs #15.
Let platform arg also include optional arch.
Fix platform+arch handling.
Add md5 sum to output for visual check of whether node binaries are different/same between runs.
Update README for platform => platform+arch switch.
Add node v0.10.32, v0.10.33 to cache list.
Merge pull request #17 from mapbox/arch

Switch platform arg to platform+arch