Repeatable field additions and minor refactoring.

This is a slightly more generic approach to making fields repeatable that doesn't require as much custom code for each field. A check of the `repeatable` property is done initially in `cmb_Meta_Box::show_form` and calls the `cmb_Meta_Box_types::render_repeatable_field` method if it is set to true instead of directly calling `$field['type']`.

Modifications made in `cmb.js` to accomodate repeatable datepicker, timepicker, and file fields.

Some fields still don't work as repeatable - colorpicker (changes needed in javascript for cases where `wpColorPicker` function not defined), radio and checkbox based fields (need to figure out a way to handle name attribute and groups), file_list, text areas / wysiwyg. oEmbed field partially works.