Enhancement: allow multiple sources

[pillarsdotnet]

I've forked and locally modified the archive module to accept an array for the source attribute, and fetch each URL in turn until success or the last one is tried, whichever comes first.

This is intended to behave similarly to Puppet's built-in file resource.

I haven't figured out how to allow multiple checksum or remote_checksum attributes in coordination with multiple source attributes. Suggestions welcome.

As soon as I get around to implementing tests, I'll submit a PR.