Add helper class for creating RSTFigure

[oruebel]

• Add RSTFigure helper class to simplify creating figures in RST documents and update API forRSTDocument.add_figure
• Add RSTToc helper class to simplify creating table of contents in an RST document and ad RSTDocment.add_toc
• Define RSTDocument.__add__ to combine documents via +
• Define RSTDocument.__iadd__ to add to a document vis +=
• Define RSTDocument.__str__ RSTTable.__str__ so simplify print and conversion to string

[oruebel]

Fix #76