Reject uploads from equal or lower versions than already available in Debian

On upload, all versions of a package for the given distribution is retrieved from the official archive. If the mentors version is lower or equal, it will be rejected with the appropriate message.

If the package is not found in Debian or there is a problem retrieving the information, the upload will be accepted.

If multiple version are found for a same distribution, we take the minimum version as a simplified algorithm.

Closes #98 (closed)