Some New Icons

Adds a few new textual icons for RDF/XML, NTriples and Turtle.  You may want to clean the positioning and sizing of these up a little bit as they don't quite match the textual sizing of the other icons but I am not fluent enough with Inkscape to do any better

Btw the issue with PNG cropping appears to be tied to the SVG - PNG conversion on my platform (OS X) so you will likely want to regenerate the PNGs yourself and commit again after merging this request.