Object arithmetic

[felixroos]

with this PR, it's now possible to do things like:

n("c3").add(n(2))
n("c3 f3").off(1/4, add(n(12)))
s("hh*2").speed("1 1.2").off(1/8, add(speed(.2)))

This did not work before because the number conversion was not working for objects. I changed it so that the conversion happens after all that object stuff, right before the arithmetic functions are called. I also simplified asNumber, because it is now not needed as often.