bugfix: iss#127 Shape.text_frame fail on no txBody

XML for a p: namespace txBody requires both p and a nsdecls
release: prepare v0.5.2 release
Merge branch 'master' into escape_series_name
escape `<c:v>` text of categories
Merge remote-tracking branch 'origin/escape_series_name' into chart_cat_multiLvl