Fixed for node v4

fix: 'install latest' works
fix: current version arch works

This is untested because the build chain isn't one I'm prepared to deal with.

fix issues related to showing the arch of installed node

Gtihub is a apparently a little picky about the syntax of this file name.

Add a local temp directory to prevent problems with cross-partition permissions. Closes #46.

download and unzip npm in a temp directory under the nvm root. fix issue
#46

Updated arch recognition for Windows 8.1

Added note about iojs

Now checks if input version is greater than latest and alerts user accordingly. Closes #44 

Add a Gitter chat badge to README.md

Added thanks and alternatives.