Improve README (#230)

- Center title to match the logo
- Change logo image to the PNG version, as SVG seems to be unsupported by PyPI:
  https://pypi.org/project/mwclient/
- Remove Landscape.io badge, as the service doesn't seem to be working:
  - https://landscape.io/github/mwclient/mwclient
  - https://github.com/landscapeio/landscape-issues/issues
- Add alt attribute to logo image