Classes preserved in SVG output and a bunch of cleanup

-A lot of cleanup as discussed in emails and #62 (commit 047830b)

-Readings selected using the xpath input are now preserved as class names. There is support for nested readings as well, though as I'm writing this up, I'm fairly certain that's impossible with the current means of xpath input. If you want me to change this to a single extra class, let me know.

More information on how this works in the notes for commit cd6265b.
I'll add a new MeiInput::GetRdgClass function that returns the rdg source as a string. This will then be passed into DocObject::AddRdgClass which adds a string to the vector.

That sound better?