CFMEException base class

Added a NavigationError exception, too...for The Future!
pytest.sel namespace = cfme.fixtures.pytest_selenium
pep8 imports
handle_popup becomes handle_alert in pytest_selenium

It also gains new superpowers, like the ability to change its wait time
and the ability to squash errors
force_navigate function, and related pytest mark
Merge pull request #402 from seandst/pytest-browser-namespace

force_navigate/go_to mark
[Table] is_displayed property
Merge pull request #406 from mfalesni/table_visibility

[Table] is_displayed property