Allow to build GUI with custom rpc path modifier

Like: /perun-rpc-devel/jsonp/ --> /perun-rpc-devel-zlamalp/jsonp/
Just pass -Dgui.url.modifier=[value] to maven during build.
Merge pull request #26 from zlamalp/gui

Allow to build GUI with custom rpc path modifier
Added possibility to change perun's configuration files location on build or runtime.

 - added variable ${perun.conf.custom} which is not used by maven (but can be)
 - running jar/war with -Dperun.conf.custom=/home/user_name/.perunv3 will override default config from /etc/perunv3
 - changed way of wiring property file in perun-registrar
 - added definition of coreProperties to perun-core (not yet used to loading properties)