rewrite tests to pytest

[vlcinsky]

This is proposal, not a bug.

Currently, the only way to run the test I am aware of is by $ python setup.py test.

I do not know, if one can run the test selecting only specific test (what would speed up development of particular features).

Rewrite to pytest would result in:

• moving all existing tests into tests folder (preserve all existing test cases)
• more modular test suite code
• nicer problem report in case of failure (nicely coloured, more expressive)
• easy option to discover existing tests without running them
• easy option to run just particular test
• allow coverage report

I am asking for an opinion before I dive into it as it takes few hours of work.

[jorgebastida]

After #69 we are ow using pytests :D Thanks!