New: GetSelectedReading to replace functionality of ReadMeiApp

 -Added GetSelectedReading function to select the user's chosen <rdg> element or the default if none is chosen.

-Added code to ReadMeiLayerElement to allow <app>/<rdg> elements as children of layers ( http://digitalduchemin.org/mei/DC1022.xml for testing )

-Added prototype code to replace ReadMeIApp in ReadScoreBasedMei but did not enable yet.
New: refactored ReadMeiLayerElement

-Condensed the for loop surrounding every call to ReadMeiLayerElement into the function itself. Renamed to ReadMeiLayerChildren to more accurately represent what it does.

-Fixed small typo in layerelement.h
New: split ReadMeiMeasure into Children function

Mimics pattern found in ReidMeiLayer(Children) so it can be called again if a child is an <app> element.
Fixed: bug where staff labels were not drawn if one was not found

Changed "return" to "continue" in view_page::DrawStaffDefLabels
Fixed: (Re)SetBrush/Pen moved to parent, code formatting

-Pen/Brush now use m_ before members and use uppercase camel for methods.

-Moved (Re)SetPen/Brush functions to device context parent class from svg/bb children
Fixed: removed legacy code

Refs #62
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
Merge branch 'develop' of https://github.com/rism-ch/verovio into develop

Conflicts:
	src/svgdevicecontext.cpp
Fixed: moved pugixml to iomei.cpp from object.cpp, reverted m_svgNodeStack to std::list

-Reverted m_svgNodeStack back to std::list

-DocObject::ReadRdgClass now only adds the string to a vector of strings instead of reading the pugi::xml_node

-MeiInput::GetRdgClass now reads the class off and passes it to the DocObject.
Merge pull request #64 from DDMAL/develop

Classes preserved in SVG output and a bunch of cleanup