Adding SHA-1/256 sum checks on exact file downloads

Hi
For the cases where "exact_filename" is being used for a download, I have added a SHA-1 and SHA-256 sum checking option. This is totally optional and will not break any existing modules. If provided it will check the downloaded file with the given SHA-1 and/or SHA-256 checksums to verify that the file downloaded is the one expected to be downloaded and not something else.

This is useful if Alien::* packages using Alien::Base are being distributed for various OSes and need to be cryptographically checked that there are no man-in-the-middle attacks being done or that the download is not corrupted.

Thanks.