Allow specifying the NPM version via a marker or environment variable.

[phw]

This is an attempt to solve #22. A npm version can be set either via the NPM_VERSION_URL environment variable or by setting the .openshift/NPM_VERSION_URL marker.

Two things might be worth discussing:

1. I also allow setting the npm version via an environment variable. This is a direct result of Problem with auto scaling #23, not sure whether you want to include this.

2. In theory there is no need to use a semver.io URL, npm install could resolve the version. So instead of setting an URL we could just allow setting the version and npm would handle something like npm install -g [email protected]. The reason I chose the URL approach anyway are:

   a) It is consistent on how the node version gets resolved
   b) It is more flexible, e.g. I could run my own service where I can configure the node and npm version to use for all my gears

[icflorescu]

Thanks, Philipp, and sorry for not being able to reply sooner. This definitely looks interesting and will hopefully pull it tomorrow. I'll also make sure to update the README.