linting standard

[owocki]

Choose a standard and start enforcing it so that the code does not become a mess, style-wise.

pep8?

[igorpejic]

We could close this, since pep8 is being forced by travis CI.
Max-line length has been agreed to 120 and can be changed in tox.ini.
Every other linting standard can also be applied in tox.ini.