Introduced ChecksumType: an enum for all supported checksum types.
(Currently MD5 and SHA-1.)
Introduced Checksum: a class that ties together a checksum type and a
value of that type to be checked.
Introduced DownloadItem: a class that ties together an URL and,
optionally, a Checksum instance (or None).
DownloadCenter now takes a list of DownloadItems.
Merge pull request #45 from Tinche/checksums

Introduced ChecksumType: an enum for all supported checksum types.