Tox oriented testing

* Added tox configurations to run testing under different environments
* Added the following tox environments configurations:
  - py27: run unit tests with Python2.7 (default)
  - pep8: run flake8 (default)
  - integration: run integration tests
  - coverage: run unit tests with coverage
  - pylint: run pylint analizer
* Added run_tests.sh script for ease tests running
* .gitignore updated to skip tox and coverage related files
* Existing tests are considered as integration test, so there are two
  test types under 'tests' folder now: integration and unit
* Added simplest unit tests