Test npm on the CI

[Fishrock123]

As of around nodejs/node#3599, make test-npm works even without node or npm in the PATH.

npm's legacy tests still require internet, but as a first step, I suggest we disable those and run the tests that don't require internet. (The tap tests I believe.)

The windows tests are apparently in pretty bad shape last I heard, so I recommend we start with OS X and then Linux.

cc @othiym23, @zkat, @iarna, @jasnell?

[maclover7]

Should this be combined with #317 or closed?

[gibfahn]

Yep, closing as a duplicate of #317 as that one has more info in it.