Multiple changes, see details.

* OFObject getMem: renamd to getMemWithSize:.
* OFString compare: renamed to compareTo:.
* Exceptions don't throw itself anymore.
* Exceptions include an error string now.
* Exceptions now got raise and string.
* New methods for OFFile:
    * changeModeOfFile:toMode:
    * changeOwnerOfFile:toOwner:andGroup:
    * delete:
    * link:to:
    * symlink:to:
* OFFile isEndOfFile renamd to atEndOfFile
* OFNotImplementedExeception newWithObject:andMethod renamed to
  newWithObject:andSelector:.
* Tests updated accordingly.
* TODO list added.