claytext: make paragraph_types, htmlescape_rules constants
claytext: fix Paragraph.is_first?

8b0cea8 (*: comments along with minor cleanup, 2013-02-18) introduced
a regression where Paragraph.is_first? wasn't being properly defined.
Fix this.
claytext: make paragraph_rules a constant
claytext: move is_* method-generation to Paragraph.initialize
claytext: fix some comments