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*
Make parser accept a char*
Merge pull request #210 from nwellnhof/nwellnhof_141122

C API fixes, additional tests, stackless rendering