document node dependency, don't hang silently if not installed

[jfirebaugh]

If you don't have node installed, make test-* hangs silently. Instead it should output a reasonable error message. Also, the node dependency should be mentioned in the README.

[mikemorris]

I swear we dropped the node dependency at some point and rewrote a few scripts in Python instead (which is needed already for gyp) so it wouldn't be needed.

[kkaefer]

The tests need node to run a http server with specially crafted responses.