Add custom field uploader for sponsor banners

[mattstratton]

For initial release, the banners will need to be uploaded "by hand" via the media library and then the URL pasted into a custom field. Eventually this should be something that can have an upload button on the episode page itself.

Possible assistance from this post http://code.tutsplus.com/tutorials/creating-custom-fields-for-attachments-in-wordpress--net-13076

[mattstratton]

This is moving along - using the super awesome CMB2 code, this is way easy to do