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.