Add MD5 support

Hello,
To make sure the downloaded file is not corrupted you could use the "md5" tag in the json file from curse to check the md5 of the downloaded file

Hi @7osCraft, this was the main reason I forked the project. You might want to switch to my fork, since this one is outdated/inactive.
You can find it here: https://github.com/timbru31/updater
(and ref for the commit which introduces MD5 sum checking: 8f7d4a9)