lr-wpan: (Fixes #636) Ext address, short address and manual assoc adjustments

This MR is intended to fix a series of inconsistencies found in the MAC. This MR should fix the issue reported in #636 (closed) and is related to #307, although it does not address this last issue because further changes might be required in sixlowpan in order to change LrwpanNetDevice::GetAddress() and LrwpanNetDevice::Send().

First, the existence of 64 extended addresses (EUI-64) is currently not enforced in the ns-3's lr-wpan MAC. This is a problem because elements of the MAC and higher layers expect these addresses to be there at all times since they are burned into the chip. On the other hand, short addresses are assignable addresses that may result from an association.

In other words, short addresses might not even exist and the MAC could work with only extended addresses. Unfortunately, the current ns-3 assumption is the opposite of this. Short addresses are handled as necessary while extended addresses are optional.

Adjustments in the LrWpanNetDevice and the LrWpanHelper were necessary to minimize this problem. The impacted examples by these adjustments were also fixed.

lr-wpan-bootstrap.cc was modified to exemplify the use of association with short address assignation and without it (extended address mode).

Further adjustments are necessary for the LrWpanNetDevice to ensure that short addresses cannot be used without a proper association (i.e without pan id and extended address). Still, the current MR should minimize this problem while also allowing for more complex manual associations, for example, multi-hop PANs like those used in mesh:

PAN coordinator--------> Coordinator------------->EndDevice