add generators and delegated generators benchmarks

just found out there's `yield* generator` which is supposed to be faster - and it is! so i added a benchmark:

![screen shot 2013-11-09 at 2 26 03 pm](https://f.cloud.github.com/assets/643505/1507577/1c49fdc6-498e-11e3-88b6-01228a2ada9e.png)

now the problem is that if i set the `mintime` too high, i'll get the following error:

![screen shot 2013-11-09 at 2 28 10 pm](https://f.cloud.github.com/assets/643505/1507578/3ae989e0-498e-11e3-8f2c-1500327e7475.png)

thus, i just made it an environmental variable and set it to `1` millisecond by default. i'm not sure why that happens.