support for MD5 checksum

Oracle provides MD5 checksum for jdk downloads: http://www.oracle.com/technetwork/java/javase/downloads/java-se-binaries-checksum-1956892.html
The cookbook currently only supports SHA256. This pull request parses the format of the checksum and automatically uses SHA256 or MD5 to verify the downloaded file. That way the checksum provided by oracle may be used directly, while maintaining support for SH256.