changed function buildPortalFrame

it now removes blocks at both sides of the portal to make sure there is
enough space for teleportation
bugfix

variables y and z now have the right values
changes in function openPortal

- the function now only searches obsidian blocks for the frame
- moved the call of createReversePortal from openPortal to createPortal
- renamed the variable which contains the location of the portal (from
'pl' to 'portal_location')
- bug: on some worlds the checking for the portal frame fails which
prevents the portal from being opened
- also removed some comments in MultiNether.java
bug fixes and improvements

CmdExecutor:
  - new function getReverseLocation: searches a location for a new
reverse portal and checks if it's safe
  - new path in portal confing and reverse portal config that contains
the id of the corresponding portal
- bugfix in createReversePortal: now it uses the right world name for
saving as 'linkto' in config

NetherPortListener:
  - now using methods from CmdExecutor to get the right destination
location for teleportation
  - bug: when in survival mode, the portal gets closed after entering