Normalise MathJax.Callback to MathJax.CallBack

I noticed that callbacks had two names and reading the documentation I assumed CallBack is the preferred one (or newer?). Anyway this normalises the two names to the camel case CallBack. This only changes the unpacked sources because I didn't find the info on how to build the packed version. The only test I ran was by changing `test/index.html` to load the unpacked sources and it went fine.