Added addMarkedFields and removeAllFields functions.

I have a website with dynamic forms whose fields can change completely in response to user input. This means removing all existing fields and adding a bunch of new ones, which can become a real hassle to do via individual removeField() and addField() calls. This commit adds removeAllFields() and addMarkedFields() functions as shortcuts for these operations: removeAllFields() calls removeField() for every field previously added, while addMarkedFields() calls addField() for every form field containing data-bv-* markup.