Added new formula: nexus

Use as proxy/cache for http://registry.npmjs.org !

See https://registry.hub.docker.com/u/marcellodesales/nexus-npm-registry
and http://books.sonatype.com/nexus-book/2.10/reference/npm-configuring.html

Must create a npm proxy repository, and a npm-all group in nexus. Then use npm install grunt --registry=$DOCKER_IP/nexus
or
adapt  ~/.npmrc