oxml: add CT_P.style setter
tmpl: add default styles to default.docx
body: add _Body.clear_content()
oxml: add CT_Body.clear_content()
docs: document CT_Styles analysis
docs: reorganize documentation
docs: add RELAX NG version of schemas
opc: transplant opc module from python-pptx

Necessarily a big hairy commit, made somewhat worse because it entailed
also converting from objectify to etree and updating most of the
unitutil mocking functions. Most of the files are new, so at least that
makes things a bit simpler.

This brings the packaging logic local, removing the dependency on
python-opc. This is to avoid package syncronization problems, at least
until the packaging logic becomes boringly stable.
docs: document table feature analysis