Make AbstractByteString public for benefit of external implementations
Fix bug in static subString of ArrayByteString
Implement RopeByteString.subString
Update unit tests
Improve javadoc for at/unsignedAt in ByteString for consistency
Add methods and Javadoc to ByteStringBuilder
Add builder(ByteString) method to ByteStrings
Merge branch 'master' of github.com:kbolino/ByteString