Unicode strings + fixing the build under linux gcc 4.8

I add support for the unicode strings in binary plists, as found in a number of iOS apps: Youtube, Google Maps... I do not add another string type for utf-16 values, but instead I convert to utf-8 string which is the same approach taken by libplist ; hence unicode strings are only read, not written. This is sufficient to me and probably also 99% of programmers.