reorg: create pptx.oxml sub-package

Moved oxml.py to oxml/util.py as a first step in breaking out oxml.py
into separate modules.
reorg: extract oxml.coreprops module
reorg: extract oxml.text module

Moved some utility functions to __init__.py, possibly temporarily.
reorg: extract oxml.table module
reorg: extract oxml.shape module
reorg: extract oxml.graphfrm module
reorg: extract oxml.picture module
reorg: extract _Element to oxml.Element
reorg: extract oxml_fromstring to oxml.oxml_fro...
reorg: extract oxml_parse to oxml.oxml_parse
reorg: remove unused new() from oxml.util
reorg: remove spec.qtag(), use qn() instead
reorg: move namespaces() to oxml package
reorg: move nsmap dict to oxml package
reorg: remove unused sub_elm() function
reorg: extract qn() to oxml package
reorg: remove unused _child_list() function
reorg: extract get_or_add() to oxml package
reorg: extract child() to oxml package
reorg: extract oxml_tostring() to oxml pkg