Create the high-level API for adding footnotes. Depends on #1.

• document.footnotes property (Footnotes collection)
• document.footnotes.add(paragraph, run, text) — creates a footnote, inserts a w:footnoteReference in the specified run, returns a Footnote object
• The footnoteReference run should use the FootnoteReference character style
• Footnote object with .paragraphs, .footnote_id, .add_paragraph(text, style)
• Footnotes collection is iterable and supports len()

Files: docx/footnotes.py, docx/document.py