README: eh! one more badge
travis: try to make coverage work as expected
travis: fix how coverage is done

There is no need to erase or combine coverage tests. For each
environment, we only run tests once. Let's see what coveralls thinks of
that.
travis: specify source file for coverage
travis: fix a mistake on options ordering for coverage
conf: add a `load()` method to the `Conf` object

Also, don't automatically create a `Conf` object (this will help
unittesting later).
conf: add unittests for configuration handling
conf: fix test to work with Python 3.3
docs: more instructions on how to install libsmi