EntryInfo#path should be String.

close #1.
Error should be raised when nil is passed to extract_data as a 1st argument.

Closes #2.
Add SevenZipReader.open_file and SevenZipWriter.open_file.

Closes #3.
Add RDoc comments.

Closes #4.
Support Rubinius 2.2.1.

Closes #5.