Adds completions for NPM (node package manager)

When not present attempts to type

npm <tab>

Results in error message on OSX when using zsh-completions package installed via homebrew

npm egrep: empty (sub)expression

Others have reported this issue here: http://www.zsh.org/mla/workers/2012/msg00744.html and here: https://github.com/npm/npm/issues/1976

This completion file is the result of executing "node completion" on the latest stable version of nodejs at time of writing (v0.10.29)