Add examples similar to the ones in the original C++ code

[kevin-keraudren]

For instance: https://gist.github.com/kevin-keraudren/a8accc61bd5ee0ebb420

Which gives with anno1.ppm and im1.ppm from the C++ example folder:

[lucasb-eyer]

Nice, thank you!

Would you mind either opening a PR which adds this file, or explicitly allowing me to add your file to the repo under MIT license?

[kevin-keraudren]

Hi, I noticed at the end of the week that the example was not quite right, hence I made another pull request to fix it: https://github.com/lucasb-eyer/pydensecrf/pull/5

With this fix and the default settings, the output for the image above becomes:

Explanations and a better highlight of the impact of the fix is presented in the new PR.

[lucasb-eyer]

Fixed by #4