More tree hierarchy checks and tests
Set defaults for new headers and lists
Rearrange get_type tests
Tests for node_previous, node_parent, node_unlink
Fix and test node_check
Fix debug flags
Stackless HTML rendering

Now that every node has a parent pointer, it's possible to implement
the HTML rendering functions without render stacks and any dynamic
memory allocations.

This commit also adds some minor optimizations that eliminate some
strbuf_put* calls for the common case and avoid printf for headers.
Fix hierarchy test
Make render_html return a char*
commonmark.rb - added walk, an iterator that walks the AST.
Make parser accept a char*
Added examples of using walk.
Merge pull request #210 from nwellnhof/nwellnhof_141122

C API fixes, additional tests, stackless rendering
Revised benchmark procedure.

Now we take the difference of the time to process the input
and the time to run with no input.  This compensates for
slow startup time in dynamic languages.

See comments on 2dcef8a
Fixed typo.

starting
commonmark.rb:  Added 'transform' and demos.
Added an Authors section to README.
Added example to spec of shortcut link with following space.

See #214.
Fixed #214 C and JS implementations.

They were gobbling whitespace after shortcut reference links,
e.g.

    [foo] bar

    [foo]: url

Closes #214.
Adjusted formatting of test - CR btw `<li>` and `<hr />`.

This is consistent with treatment of other block elements
in list items.