parse RST quoted literal blocks

closes #65
RST quoted literal blocks are the same as indented literal blocks (which
pandoc already supports) except that the quote character is preserved in
each line.

This includes test cases for the quoted literal block, as well as
additional tests for line blocks and indented literal blocks, to verify
that these are unaffected by the changes.
parse RST class directives

The class directive accepts one or more class names, and creates a Div
value with those classes.  If the directive has an indented body, the
body is parsed as the children of the Div.  If not, the first block
folowing the directive is made a child of the Div.

This differs from the behavior of rst2xml, which does not create a Div
element.  Instead, the specified classes are applied to each child of
the directive.  However, most Pandoc Block constructors to not take an
Attr argument, so we can't duplicate this behavior.
Merge pull request #1781 from bergey/rst

parse RST quoted literal blocks and class directives