[ResourcePriorities] Spec Updates

Based on mailing list and conference call feedback, I have made the following changes to the Resource Priorities editor's draft [1]:

*         Removed extension declarations from each partial interface
*         Removed 'readonly' from IDL attribute definitions
*         Updated Lazyload MUST clause updated to SHOULD
*         Defined content and IDL attribute definitions for lazyload and postpone
*         Clarified that readyState IDL attribte and DOMContentLoaded events are not impacted by lazyload or postpone
*         Clarified that if no lazyload elements exist, lazyloaded event fires immediately after load event
*         Added 'resource-priorities' CSS property
*         Postpone only impacts audio element if controls boolean attribute is specified
*         Parser will not block launching new scripts on lazyloaded stylesheets
*         Rendering will not be blocked on lazyloaded stylesheets
*         Added SVG image, use, script, and tref elements
*         Added SVG 1.1 reference
*         Lazyload and postpone have no effect on svg feImage when externalResourcesRequired is true
*         Updated attribution section

Apologies for the late Friday afternoon updates. I will close loop directly on all the mail threads where the feedback was given.

Thanks,
Jatinder

[1] https://dvcs.w3.org/hg/webperf/raw-file/tip/specs/ResourcePriorities/Overview.html