Constructors for creating multi-tracker torrents

There is multi-tracker support in library, but it is impossible to create such torrents.
This patch partially fixes it (didn't come with a good way to pass tier description from command line, so this was left unimplemented).

If you have in mind better way (than 5 different constructors) for providing a support for both `file` vs `files` and `announce` vs `announce-list` feel free to suggest it.

Thanks!