Fix for document.body() being null

[energyuser]

That is in case you implement this: https://github.com/cameronmcefee/plax/issues/66#issuecomment-119740283 Or you could execute the script using

$( document ).ready()

in plax.js itself, which might be even better. Your call @cameronmcefee

[cameronmcefee]

in plax.js itself, which might be even better

I'll have to take a look to get my head around it (haven't looked at the code in a year), but having it in the JS itself seems like a good way to go.