clayoven.gemspec: bump to version 0.2
claytext: match multi-level :emailquote
claytext: remove extra newlines, fix minor typo
claytext: introduce Paragraph#level for :emailquote type

"> " is a level 1 :emailquote, ">> " is a level 2 :emailquote, and so
on.
claytext: introduce Paragraph#subheading

Along with levels, this can be useful for a Wiki-like website.
claytext: remove Paragraph#is_first? hack

We don't use it in the template anymore.
claytext: use Ruby 2.0's %i for symbol-list creation
clayoven: use Ruby 2.0's __dir__

__dir__ is a drop-in replacement for File.dirname(__FILE__).
clayoven.gemspec: fix s.files
clayoven.gemspec: add README.md, LICENSE to extra_rdoc_files
clayoven.gemspec: change post_install_message to point to README.md
claytext: strip ^four-spaces in :codeblock
claytext: minor grammar nit in comments
clayoven: introduce special "hidden" topic

This topic doesn't have an index, and should contain pages that won't
be linked to from the site.  For example, 404.
clayoven: sort topics by timestamp too
claytext: allow only 2-level :emailquote and :subheading

Higher levels are ugly and unnecessary.
claytext: introduce /  $/ syntax for <br>

Two trailing whitespaces at the end of a line should be replaced with
a linebreak.  Taken from GitHub-flavored markdown.
claytext: use / $/ to indicate <br>

Why use two trailing whitespaces when one will suffice?
claytext: introduce ^two-spaces for :listitem

Apart from the vertical inter-item spacing (which I really don't care
about), a list item simply introduces a left margin.  Let's just
emulate this when each line in a paragraph matches ^two-spaces.
claytext: fix comment for :footer rule