Fix/adjust a series of things on sessions and add comparematch command

• Add a listpool command to list all files of a certain type in the pool, filtered by session (#33 (closed))
• fix add to make it work also when tinfoilhat and aliensrc files are added separately (wrong property reason instead of select_reason) (#34 (closed))
• Command.exec() and Command._run() should give consistent input to *.run() regardless of the processing type (#35 (closed))
• commands should return actual results (and not only True) if they actually produced some result (#36 (closed))
• add dryrun option to all commands (#37 (closed))
• download deb source files also when match present (#38 (closed))
• add comparematch command (#39 (closed))
• remove old code to generate csv data from snapmatch (#40 (closed))