Change URL of GitHub website

Starting from 5th April 2013, Github changed the URL of project pages
from *.github.com to *.github.io. Although FNSS website is still
reachable at fnss.github.com, it is faster to access it at
fnss.github.io. Therefore, it is appropriate to change all references
to the new URL.
Change C++ and ns-3 Quantity::toString() signature

This commit changes the signature of Quantity::toString() function,
present in both C++ and ns-3 components, so that it now returns by
default the value and the unit concatenated without spaces.

This fixes a defect in the ns-3 adapter which was not able to
recognise capacities and delays with a space between value and unit.
Improve core examples

This commit improves the examples of the core component, by making
the difference between networkx and fnss functions clearer.

This commit also extends the rocketfuel_tm example to show how to
generate a traffic matrix with specific origins and destinations
randomly picked among topology nodes.
Validate ns-3 directory in ns-3 Makefile

This commit adds a target in the ns-3 Makefile to verify that the
NS3_DIR parameter (i.e. the directory where ns-3 is installed) is
valid.
Minor typos fixes and rephrasing in comments and documentation
Improve stability of dynamic traffic matrices generators

This commit adds code that raises errors when log_psi and gamma
parameters used for generating stationary and cyclostationary traffic
matrices cause division by 0 or other errors leading to invalid
traffic volumes.