Adapt RSS feeds to the multi-blog feature

This task is part of the blog application improvement proposal: http://design.xwiki.org/xwiki/bin/view/Proposal/BlogImprovemultiblogsupport

Existing RSS URL should be maintained (for existing subscribers)

• RSS will only work like the Blog List content defined in the proposal.
• New proposed RSS URL will point to:
  • The blog WebHome for a blog rss with a specific sheet
  • The category page for a category with a specific sheet
  • The category WebHome for "All" RSS with a specific sheet