More lenient `isGenerator()` function

TJ, this is my take at #37.

Basically all it is is making the `isGenerator()` function a bit more lax, since it's impossible for polyfill runtimes to properly satisfy the `toString.call()` check.

Additionally, I made the node executable pluggable in the Makefile, so that it's easy to test with different versions of node via:

``` bash
$ NODE=`n bin 0.11.3` make test
$ NODE=`n bin 0.11.7` make test
$ NODE=`n bin 0.11.8` make test
# etc.
```

It's pretty minimal overall so I hope you'll merge it. Cheers!