Add requirejs support

The example page (example/index.html) works as before (global swal & SweetAlert functions).
I added an example page using requirejs (example/index_requirejs.html), in which require.js is loaded instead of sweet-alert.js. When using the require function in the inline script tag, sweet-alert.js is loaded and the swal function is usable inside the require function. With requirejs loading, no sweet-alert function is declared globally.