xwiki.cfg and xwiki.properties should be read based on system encoding

xwiki.cfg and xwiki.properties are read as Java property files, which is using iso-8859-1 encoding. This is absolutely not practical neither intuitive for the end user, which by default will use it operating system encoding to store configuration file.
Moreover, we use utf-8 almost everywhere, the encoding used for properties file is a thing of the past, we should probably fix that.

See http://stackoverflow.com/a/4660195 for details about this matter.