Handle minus sign and fullwidth hypen minus correctly

Minus Sign: U+2212
Fullwidth Hypen Minus: U+ff0d
Update CHANGELOG.md
Update README.md (fullwidth hyphen and minus sign)
Bump version to 2.0.0.pre2
Use #pack instead of embedding literal to the code
Extract character_names.rb
Extract invalid_encoding_error.rb
Normalize magic comments
Extract CommonMethodsForField