Added NIFs for parsing date/time from string and back
Implemented environment variable substitution in strings

1. Replaced macro notation from ${NAME} to $(NAME)
2. Environment variable notation is enhanced to be in one of two
   forms: $env{NAME} or ${NAME}
3. Implemented environment substitution in strings
4. Implemented $path{FormattedString} function
5. Implemented $date{FormattedString} function