Merge branch 'RC_1_2' into RC_2_0

update revision

fix contributing docs

bump version to 1.2.11

fix issue where pad blocks would skip the zero-hash check and conside…

…r the last piece as passed, which in turn could cuase a hybrid magnet link to fail with inconsistent-hashes

fix issue where pad blocks would skip the zero-hash check and conside…

…r the last piece as passed, which in turn could cuase a hybrid magnet link to fail with inconsistent-hashes

bump version to 1.2.10

bump version to 1.2.10

add missing includes and missing windows-specific headers to tarball

fix missing header in test_fast_extension.cpp