tests: py.test with coverage and pep8 support

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>
global: support for Python 3

* Fixes problems with exceptions handling and str/unicode.  (closes #7)
test: support for tox

* Adds support for tox testing.  (closes #4)

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>