Fix issue where the deprecated Element#childOf was not initially present on elements.

We added it to the Element.Methods object in deprecated.js _after_ the initial call to Element.addMethods, meaning that it didn't get copied over unless the user made a subsequent call to Element.addMethods for whatever reason. Unit tests didn't catch this because they call Element.addMethods before testing starts.
Remove the event registry from an element when its last observer is removed.
For `rake dist`, grab the submodule for whatever selector engine was specified, even if it’s the default.

This is the thing I forgot to do when I removed our repo’s copy of Sizzle. That version of Sizzle _did_ get used if you tried to `rake dist` but you didn’t have the Sizzle submodule initialized. Considering that we’d have fetched the submodule automatically anyway, it’s not clear how much of a victory this was for user-friendliness.
Change the Sizzle module to pull sizzle.js from /dist rather than /src. [close #194]