Extend debugging part 2: Major changes in the extension algorithm

D2TreeCore.java
* The validity of some destinations is now checked in forwardExtendRequest (msg.getDestinationId() != RoutingTable.DEF_VAL).
* New-leaf-siblings are now connected together as neighbors in forwardExtendRequest.
* Hops are not taken into account to compute the size of a bucket anymore.
* Implemented new method for printing the state of the tree to a file.

D2Tree.java
* Temporarily removed 'unsupported operation exception' throws because they are an issue for the moment.

D2TreeMessageT.java
* Added PRINT_MSG as type.

GetSubtreeSizeResponse.java
* Set initial size to 0.
* Commented out method void incrementSize().

RoutingTable.java
* Print method added.

Misc
* All messages now know which one is the initial node that triggered them when it joined the net.
* New class: PrintMessage (used to print the tree -mostly for debugging purposes-).