Support for signing multiple files at a time

[tomasr]

First of all, let me say I've been testing Relic for our needs and have been very impressed by it. Documentation could be improved, but it only took a couple of hours to get something working.

One minor suggestion for improvement that would be useful, at least when running as a standalone signing tool would be able to provide multiple files to sign at a time (with the same signature type, of course), particularly if that can lead to performance improvements during the operation. For example, we often use SignTool passing in 16-20 files at a time, and it reduces total signing time by ~50% compared to signing them one at a time.

I took a quick look at the code and doesn't seem like the "take multiple files at a time" part would be too hard to do, though not sure about the performance part.