[ns] Announcement of error-models for wireless networks.

["jahn-isi" <jahn@ISI.EDU>]

I expanded "node-config" command to add error modules at the wireless networks' protocol stack.
Previously, ns only allows to attach error models to links not wireless media. By prvoiding the
name of a function that specifies a specific error model and initializes its related parameters as one
of node-config command's parameters, users can layer the error model upon Mac modules.

Furthermore, node-config can hook a FEC module over wireless networks, even though it is yet
experimental. You can see how they are integrated into the wireless network protocol stack by
referring to test-suite-wireless-lan-newnode-err.tcl for details. Currently you can see these modules
from ns-src-shapshot.tar without their documentation.

After finishing our experiments, we will provide vairous error models and more realistic 
FEC functions for wireless networks in addition to their related documntation.

Please if there are some bugs and questions with the current wireless error models, let me know.

-jahn