Fix false positives due to ellipses

This is primarily a fix for the largest class of false positives in ftfy. It also includes some other small fixes, and merges in the Twitter-streaming framework that I use to test ftfy. (I've run it for about 1.5 million tweets on this version, and seen no false positives.)
This docstring is somewhat out of date, in that it refers to "twice the weirdness", but the factor of two has moved into the `sequence_weirdness` function.