Adding sample, fixing minor issues and expanding Readme.

[apacha]

• Adding a minimum sample file to actually run and test the code without having to download the entire dataset.
• Removing generated polyiou.py file from the repo and added it to ignore list, because it would be changed by any user running a different version of SWIG.
• Adding SWIG install instructions for MAC.
• Fixing two issue in obb_anns.py:
  • Assuming a list of strings, but actually getting a string instead
  • Concatenating the subdirectory of the segmentation images twice, creating a path like sample/sample/segmentation/xyz_seg.png instead of sample/segmentation/xyz_seg.png

[yvan674]

Thanks!