Add a generator implementation

[austinzh]

Hi, Your multi-threading implementation give me a lot of inspirations for how to use the estimator API. After some dig-in, I found there is way to use only a simple generator instead of Queue and threading to achieve similar or even better performance(because there is not context switch in generator implementation).

Hope you guys will like it.

starting threaded
starting generator
Threaded time was 1.24210524559021; s
Generator time was 1.1133246421813965;  s
Generator was 1.115672103652073 times faster!

Austin