Add Range support to media fields

Creates a custom field widget which has a "stream" view associated with it.  This view correctly responds to partial content requests for media (via Plone's existing blob iterator), meaning seeking is now possible.  Previously, a browser would need to download the entire video before this would be possible (or in Chrome's case, refuse to see at all).

Adding support for content ranges also ensures that iOS can view videos (see documentation requiring [byte-range support](https://developer.apple.com/library/safari/documentation/AppleApplications/Reference/SafariWebContent/CreatingVideoforSafarioniPhone/CreatingVideoforSafarioniPhone.html#//apple_ref/doc/uid/TP40006514-SW6))