#42 P7S detector

[Giacomo Boccardo]

I've implemented a P7S detector using the following rationale:

• P7M: PKCS#7 with a non-empty content;
• P7S: PKCS#7 with an empty content.

My approach could seem quite quick and dirty, but we have to work on a stream, so…

Please review it, find a better way or adapt it to your current head :)