Makefile: add NODE custom variable support

Makes it easy to plug in a different node executable to test with.
index: check the object's constructor name for isGenerator()

This is for polyfill runtimes, since it's impossible for them to be
able to pass the native toString() test.

It's unfortunate that a lib like co needs to worry about something like this,
but this is JavaScript and if it looks and quacks like a duck…
Merge pull request #38 from TooTallNate/add/regenerator-support

More lenient `isGenerator()` function