Initial script for making an RC

Initial script for building and signing release candidates.

Needs a good bit of love to clean it up and make it more functional.
- Needs to handle parameters in a more sensible manner.
- Needs to allow the user to pass the artifact name instead of mandating that the project name in git be used. This should work for ASF projects but it's not terribly flexible.
- Only a minimal number of release formats are available and there's no way for the user to customize/pick what they want.
- The artifact MD5-ing and signing is done in an extremely naive way that is easily broken if more release formats are added.