changes to the readme, and widen the verson constraints on logging

This PR contains some styling tweaks the the readme, and widens the version constraint on `logging`. `>=0.9.2 <0.9.3` is pretty narrow, and would cause problems if another package needed to import dart_coveralls (and other deps for that package had different, narrow constraints on `logging`). It looks like the reason for the narrow constraint was because of the export of the logging package; that export didn't _look_ like it was needed.

Nothing here's too critical; comments welcome!