New: readings are preserved as classes in svg output

-Added members in DocObject to keep track of associated readings

-Added reading vector (for case where multiple readings might affect one object) to svgdevicecontext

-Changed m_svgNodeStack to a stack rather than a list to match pen/brush stacks

-GetSelectedReading now appends any chosen reading sources to the direct children of the <rdg> element under the "source" attribute

-All DocObjects in iomei now save all these associated readings in a vector (in case there are multiple)

-These readings are recalled in svgdevicecontext and appended to the svg gClass parameter to startGraphic