Mention minimum node version; Fixup version checking

On my Mac, the os.tmpdir method is called os.tmpDir. Looking in the node repo, this was renamed about 5 months ago: http://gitelephant.cypresslab.net/node-js/commit/3fe6aba558efe3f1cac17dd6c0c41fb11443788b.  This CL updates mca-create.js to use either os.tmpdir or os.tmpDir depending on which is defined.

I've also added a quick note about the minimum node version required. Looking at the docs, os.tmpdir was added in 0.7.11, so, given the version check implemented, that seems like the lowest version.