Add support for Python 2.6 in core source and test

This commit should address most of issue #1 by backporting source
and test code to Pyhon 2.6.

These changes include:
* Remove dict comprehensions
* Add catch for collections Counter import
* Remove with-metadata in setup.cfg
* Change all tests so that unittest2 can be used if Python < 2.7

Still remains to change all the documentation and setup.py to mention
that now FNSS should work fine with Python 2.6
Fix defects extended_barabasi_albert. Closes #7

The extended_barabasi_labert generator had two defects:
1. It sometimes entered into an infinite loop
2. It was always failing with Python 3

This commit fixes both defects