Fix table autofit behavior and improve column width control.

Current issues

• table.autofit is broken / doesn't work as expected (upstream autofit is broken python-openxml/python-docx#209, 13 comments)
• Column widths are difficult to set reliably (upstream Column widths in MS Word python-openxml/python-docx#360, 8 comments)

API Design

• table.autofit_behavior — WD_TABLE_AUTOFIT enum (AUTOFIT_TO_WINDOW, AUTOFIT_TO_CONTENTS, FIXED_WIDTH)
• column.width — Emu value, reliable set/get
• table.preferred_width — total table width
• table.allow_autofit — bool

XML

• w:tblPr > w:tblW (table width, type=auto/dxa/pct)
• w:tblPr > w:tblLayout w:type='fixed' or 'autofit'
• w:tcPr > w:tcW (cell width)
• w:tblPr > w:tblStyleColBandSize, w:tblStyleRowBandSize

Upstream: python-openxml#209 (13 comments), python-openxml#360 (8 comments)