Add changes done in the GF repository

See: https://github.com/GrammaticalFramework/GF/commits/master/lib/src/estonian

Changes:

  - https://github.com/GrammaticalFramework/GF/commit/0a713d38af14fb4e6b4c92a67141ce45ad6f2f83
  - https://github.com/GrammaticalFramework/GF/commit/e1d18e7bb29098544230f125013a0314a32a4eb4
Update README with information on how to build PGF
Update tests