DNF API name converter

[mcyprian]

In this PR:

• code of name convertor based on DNF API query
• integration of new convertor
• unit tests

Tests are failing, because I haven't found the way how to get dnf into travis yet.

[hroncok]

Can you fix the Travis build somehow? I guess it could be quite challenging to get dnf to Ubuntu, so just disable those tests there fo now.

[hroncok]

DandifiedNameConvertor vs DantifiedNameConvertor (commit message)

[hroncok]

I know pycairo is not exactly on PyPI, but this test would not pass:

class TestDandifiedNameConvertor(object):

    def setup_method(self, method):
        self.dnc = DandifiedNameConvertor('fedora')

    @pytest.mark.parametrize(('pypi_name', 'version', 'expected'), [
        ('pycairo', '2', 'pycairo'),
        ('pycairo', '3', 'python3-cairo'),
    ])
    def test_rpm_name(self, pypi_name, version, expected):
        assert self.dnc.rpm_name(pypi_name, version) == expected

[mcyprian]

Tests requiring DNF are skipped if module is missing, pycairo package is now part of the unit tests, passing on my local machine.

[hroncok]

LGTM

[mcyprian]

Thanks, I think this PR is ready to be merged.