Add an option to use system-wide "node"

Using `-n system` flag, node.js will not be downloaded nor
compiled. The system-wide version will be used. npm needs to be
tricked a bit about using the appropriate prefix. We enforce the
prefix to be used by setting NPM_CONFIG_PREFIX variable in the
environment.
Merge pull request #41 from vincentbernat/feature/use-system-nodejs

Add an option to use system-wide "node"