Added a new option "delete_files: Boolean"

Added an option to clean_transmission "delete_files", by default is false. If set to yes it will remove the downloaded files as when removing torrents from transmission. Useful when files have already been copied and were just left behind to finish a seeding goal.
Merge pull request #257 from gandazgul/patch-2

Added a new option "delete_files: Boolean" to clean_transmission
Added virtualenv activation example in the documentation.