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)
Core: new workflow with users preferred shells

- U:D:preferredShell renamed to U:D:preferredShells
  - user can specify multiple shells which he prefers ordered by
    preference
- new module for attribute UF:V:shell
  - get shell for user on facility based on users preferences
- basic test of UF:V:shell module
Merge new workflow with users preferred shells

Based on pull request #29 from papperwing/shells
Clean up of changes in code.
Renamed module U:D:preferredShell to U:D:preferredShells.
Merge pull request #28 from zlamalp/devel

Added possibility to change perun's configuration files location on build or runtime