advanced functionality changes

CmdExcecutor.java:
  - added global logger variable
  - added new createPortal method for future event handling of
PortalCreateEvent
  - improved method 'buildPortalFrame'
    - now actually opens the portal
    - added some blocks around the portal to be removed to provide
enough space for the player
    - now builds a platform under the portal if the location is unsafe
  - removed some unused and outcommented statements
  - some performance improvements
  - improved method 'getNearestPortal'
  - added methods for searching reverse portals
  - added separate methods for getting Portal-Ids, ReversePortal-Ids
and both Portal- and ReversePortal-Ids
  - added method for getting ReversePortals
  - fixed some smaller bugs, especially with the creation and storage
of new portals

MultiNether.java, plugin.yml:
  - changed commands

NetherPortListener:
  - add eventhandler for PortalCreateEvent to replace the portal-create
command
  - changed the event handler for PlayerPortalEvent to work more
reliably

Portal.java:
  - added CorrespondingID for linking between portals