Remove claims of Python 2.6 compatibility

This commit removes any claim of compatibility with Python 2.6 from
the core library and the ns-2 adapter. The reasons of this
incompatibility are:
* The use of dict comprehension in the core library
* The use of the argparse library in ns-2 adapter

The future plan is to remove dict comprehensions in the core library
and reclaim Python 2.7 compatibility.
Add creation of global dist archives in Makefile

This commit modifies the global Makefile dist target so that it now
creates global .zip and .tar.gz archive of the entire library.
Fix .gitignore rule for global doc and dist directories

This commit fixes the rule for excluding the root doc and dist
directories.
Fix problem with TopoZoo topology parser

This commit fixes a defect occurred when trying to parse topologies
from the Topology zoo dataset using NetworkX 1.6.

This commit fixes the problem in the code and adds a test case for
Topology zoo parsing which was missing. It also increments the version
of the core library to 0.1.2.