Clarify scripting support contexts

[mattgarrish]

EPUB currently requires support in container-constrained contexts (iframe, object, embed) and leaves spine-level scripting optional.

EPUB 3.1 makes the following changes:

• container-constrained scripting is limited to iframe
• container-constrained scripting becomes a should
• spine-level scripting in fixed-layout documents becomes a should
• spine-level in reflowable documents becomes a should if the documents use the scrolled-doc or scrolled-continuous interfaces defined by the rendition:flow property
• if spine-level scripting is support for reflowable documents, the scrolled-doc interface must be supported and scrolled-interface should be supported

Whether scripting is a "should" or "must" may change in a future draft if a security and privacy model are worked out.

For more information, see the scripting group's working document.