Introduce 'autoinit' feature

Enable the automatic creation of iframes when the script tag which
included Pym.js contains a `data-pym-autoinit` attribute. Ensure that
duplicate iframes are not created in cases where pym.js is included more
than once in the same document.
Merge branch 'autoinit' of https://github.com/jugglinmike/pym.js into jugglinmike-autoinit
House style and bug fixes.