fix incompatible `Counter` usage on Python 2.6

Python 2.6 does not have a `Counter` module, this changeset not only adds a backported implementation but initiates a test harness to fix the lack of testing for different Python versions.

A `tox.ini` file is added that implements what is needed for basic testing of `ceph-brag` which includes pep8 checks (and fixes for the pep8 failures).

There is currently no `make check` target to run these, we should wait until the other Python scripts are ready. For now, these tests allows us to have some basic testing of different environments.

    $ tox -r
    GLOB sdist-make: /Users/alfredo/python/upstream/ceph/src/test/python/brag-client/setup.py
    py26 recreate: /Users/alfredo/python/upstream/ceph/src/test/python/brag-client/.tox/py26
    py26 installdeps: pytest
    py26 inst: /Users/alfredo/python/upstream/ceph/src/test/python/brag-client/.tox/dist/ceph_brag-0.1.zip
    py26 runtests: commands[0] | python setup.py develop
    running develop
    running egg_info
    writing requirements to ceph_brag.egg-info/requires.txt
    writing ceph_brag.egg-info/PKG-INFO
    writing top-level names to ceph_brag.egg-info/top_level.txt
    writing dependency_links to ceph_brag.egg-info/dependency_links.txt
    reading manifest file 'ceph_brag.egg-info/SOURCES.txt'
    writing manifest file 'ceph_brag.egg-info/SOURCES.txt'
    running build_ext
    Creating /Users/alfredo/python/upstream/ceph/src/test/python/brag-client/.tox/py26/lib/python2.6/site-packages/ceph-brag.egg-link (link to .)
    Adding ceph-brag 0.1 to easy-install.pth file
    
    Installed /Users/alfredo/python/upstream/ceph/src/test/python/brag-client
    Processing dependencies for ceph-brag==0.1
    Searching for requests==2.3.0
    Best match: requests 2.3.0
    Adding requests 2.3.0 to easy-install.pth file
    
    Using /Users/alfredo/python/upstream/ceph/src/test/python/brag-client/.tox/py26/lib/python2.6/site-packages
    Finished processing dependencies for ceph-brag==0.1
    py26 runtests: commands[1] | py.test -v
    ============================================================================================================ test session starts =============================================================================================================
    platform darwin -- Python 2.6.8 -- py-1.4.20 -- pytest-2.5.2 -- /Users/alfredo/python/upstream/ceph/src/test/python/brag-client/.tox/py26/bin/python2.6
    collected 1 items
    
    tests/test_ceph_brag.py:9: TestCephBrag.test_basic PASSED
    
    ========================================================================================================== 1 passed in 0.09 seconds ==========================================================================================================
    py27 recreate: /Users/alfredo/python/upstream/ceph/src/test/python/brag-client/.tox/py27
    py27 installdeps: pytest
    py27 inst: /Users/alfredo/python/upstream/ceph/src/test/python/brag-client/.tox/dist/ceph_brag-0.1.zip
    py27 runtests: commands[0] | python setup.py develop
    running develop
    running egg_info
    writing requirements to ceph_brag.egg-info/requires.txt
    writing ceph_brag.egg-info/PKG-INFO
    writing top-level names to ceph_brag.egg-info/top_level.txt
    writing dependency_links to ceph_brag.egg-info/dependency_links.txt
    reading manifest file 'ceph_brag.egg-info/SOURCES.txt'
    writing manifest file 'ceph_brag.egg-info/SOURCES.txt'
    running build_ext
    Creating /Users/alfredo/python/upstream/ceph/src/test/python/brag-client/.tox/py27/lib/python2.7/site-packages/ceph-brag.egg-link (link to .)
    Adding ceph-brag 0.1 to easy-install.pth file
    
    Installed /Users/alfredo/python/upstream/ceph/src/test/python/brag-client
    Processing dependencies for ceph-brag==0.1
    Searching for requests==2.3.0
    Best match: requests 2.3.0
    Adding requests 2.3.0 to easy-install.pth file
    
    Using /Users/alfredo/python/upstream/ceph/src/test/python/brag-client/.tox/py27/lib/python2.7/site-packages
    Finished processing dependencies for ceph-brag==0.1
    py27 runtests: commands[1] | py.test -v
    
    ▽
    remove unused variables, gets all tox envs passing
    ============================================================================================================ test session starts =============================================================================================================
    platform darwin -- Python 2.7.5 -- py-1.4.20 -- pytest-2.5.2 -- /Users/alfredo/python/upstream/ceph/src/test/python/brag-client/.tox/py27/bin/python
    collected 1 items
    
    tests/test_ceph_brag.py:9: TestCephBrag.test_basic PASSED
    
    ========================================================================================================== 1 passed in 0.09 seconds ==========================================================================================================
    flake8 recreate: /Users/alfredo/python/upstream/ceph/src/test/python/brag-client/.tox/flake8
    flake8 installdeps: flake8
    flake8 inst: /Users/alfredo/python/upstream/ceph/src/test/python/brag-client/.tox/dist/ceph_brag-0.1.zip
    flake8 runtests: commands[0] | flake8 --select=F ceph_brag.py
    __________________________________________________________________________________________________________________ summary ___________________________________________________________________________________________________________________
      py26: commands succeeded
      py27: commands succeeded
      flake8: commands succeeded
      congratulations :)