Add new line type elements: horizontal line (hr) and page break

It will be very useful to create two new line types:

• the horizontal line, equivalent to the <hr> html tag
• a page break, similar to google docs (Ctrl+Enter), and mainly used when printing/exporting waves (PDF)

More related info:
http://wave-protocol.googlecode.com/hg/spec/conversation/convspec.html#anchor22
http://www.google.com/support/forum/p/wave/thread?tid=19eb3bb679aea71a&hl=en