pugixml incorporation

* Changed everything in svgdevicecontext from string-based to pugixml-based and deprecated all irrelevant functions.
* Merged master changes since 2014-12-05 into fonts branch
* Updated "make install" to copy everything from /data/ into /usr/local/share/verovio; I had it do everything instead of the specific Bravura and Leipzig subfolders because a) it's easier with cmake's INSTALL() syntax and b) in case more default fonts are added in the future. Let me know if you want me to change this.

Not sure why this can't be automatically merged and that's sorta scaring me.