version bump 0.7.2: bughunt

- read BOM, handle UTF16LE-encoded XML
- handle namespaces in [Content_Types].xml
- parse workbook rels to determine sheet files
- numbers OSX boolean support (apparently requires "0" or "1")
- XLSX force "General" style to be serialized, omit implied cell type and style
- updated SSF to 0.7.0 (h/t @sysarchitect)
- updated jszip to 2.2.2
- removed old tests/files path, replaced with test_files
- themes written
- ignore potential existence of thumbnail when calculating relationship ids