fix deps, improve develop and test workflow
    
    * Fix dependencies in requirements.txt: we need `fedmsg[consumers]`, not
    just `fedmsg`, and add some missing ones
    * Remove deps from `setup.py`, because it was not up-to-date anyway and
    there's little sense to keep deps defined there as well, as long as we
    don't publish this on PyPI
    * Make all Makefile targets phony (defined at the very same place, so
    that it's not forgotten next time we add a target)
    * Print test coverage when the test suite is run
    * Use `virtualenv --ignore-installed` to make sure all the deps possible
    are installed into virtualenv, regardless of system libs.
    * Extend instructions how to set up a development environment
    * Add testing instructions
    * Use the same test approach in spec as we do in development
    
    Differential Revision: https://phab.qa.fedoraproject.org/D1123