add find method to generators

[ssartell]

This pull request adds a basic find method to all generators.

[atavakoli]

Since generators may theoretically yield an infinite number of terms, can this lead to an infinite loop? Could there be a max number of iterations also passed in, where it would return undefined if the search didn't succeed within the specified number of terms?

I don't think this is an actual issue with any of the existing generators, which all generate a finite amount of terms, and none of the other iteration methods deal with this. But for very large sequences, having a way to exit early would be useful to keep applications responsive, without resorting to WebWorkers.

[ssartell]

While I agree with your point, I mimicked the other existing methods. Any flaw this method has already exists in the library. And as you said, none of the generators in the library are infinite so it should not be a problem in practice.

[dankogai]

Thank you. Travis is happy again with io.js dropped. I added the doc to README.md and incremented the version to 0.5.3.