changed portal handling and behavior

portals won't be safed in config-files anymore
no more use of static links from config file -> worlds are now linked
by their names (e.g. world1 -> world1_nether)

CmdExecutor.java:
  - added method 'getPortalsNearLocation' for searching portals like
vanilla (portal-block above obsidian-block)
  - added method 'get_NearestPortal' to find the closest portal
location received from 'getPortalsNearLocation'
  - added method 'getLocation' for finding a safe location for new
portals
  - changed method 'getReverseLocation':
    - uses the original y-value instead of a random one
    - minimal y-value is now 15 blocks below the original value

NetherPortListener.java:
  - onPlayerPortalEnter:
    - getting world and corresponding nether by their names
    - now searches for a portal in the target area instead of
retrieving one from config-file

  - onPortalCreate:
    - getting world and corresponding nether by their names
    - now searches for a portal in the target area instead of
retrieving one from config-file
    - now builds a new portal if there is no portal found in the target
area