Add engines field to package.json for samples

[JustinBeckwith]

We recently added support for engines in package.json:
http://www.marcusoft.net/2015/03/packagejson-and-engines-and-enginestrict.html

Should we pin down the supported nodejs version in each sample?

[jmdobry]

Do we have a list of the available Node versions?

[jmdobry]

Found it: https://storage.googleapis.com/gae_node_packages

[jmdobry]

Closed by #17