Remove monkey patches

This pull request removes the `Enumerable` and `Kernel` monkey patches in the Twitter gem.

I moved `Enumerable#threaded_map` into `Twitter::Util.threaded_map` and replaced all `threaded_map` calls with calls to this method on the util module. Util modules suck, but I think they suck less than surprise monkey patches to core classes.

I was able to completely remove the `Kernel#singleton_class` implementation for 1.8 as it didn't appear to be called from anywhere.