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
refactor
make the `isGenerator()` function more generic

For some background, see this Twitter conversation:
https://twitter.com/TooTallNate/status/397185683162288128

tl;dr this works with both regenerator as well as native V8 generators,
and it is probably faster in general, so hey why not. Yay JS………
fix typo in jsdoc comments
Merge pull request #39 from TooTallNate/update/is-generator-generic

make the `isGenerator()` function more generic
merge
Release 2.2.0