Shim Array.indexOf for IE8

Line 226 of command.js fails in IE8, which doesn't support Array.indexOf (yeah, clearly I have too much time on my hands).

You can see the impact of the error if you open IE8 and:

- Go to http://mathquill.com/
- Enter "x" (or any other character) into the demo mathquill editor
- Press "/" to add a new fraction
- There's a js error and the fraction isn't added.

So I've just added a shim for indexOf (based on https://gist.github.com/dhm116/1790197).

Demo of fixed version:
http://eoghanmcilwaine.github.io/mathquill-ie8-indexOf.html