Quit browsers between tests

Before, pyfuct only quits browsers on end of testcase (`tearDownClass`).
This will avoid keep multiple browsers open, that aren't not used.

In case property `reuse_browser` is configured to true, we mantain only
the default browser open and clear his session.
refactor quit browser on tearDown
Merge pull request #20 from rodrigopr/quit_browsers_between_tests

Quit browsers between tests