gracefully handle null fields

RB_ID=163651
Fix small README issue
Make latest runtime version consistent
identify enum fields as TType.ENUM

maintain backward compatibility

RB_ID=163883
maven-plugin: do not extract into subdirectories

Fail fast if extraction will overwrite an existing file with different
contents.

RB_ID=164017
do not extract if the file exists and is the same

RB_ID=164417
skip file copy from references if existing file is the same

RB_ID=164855
fix ThriftStructMetaData use camelCase method names to match generated code

RB_ID=164461
update changelog
bump version to 3.3.0