Move entry-point main methods to separate package

Using ttorrent as a library, I ran into a couple annoying visibility issues.  By moving the entry-point main methods of the codebase into classes of a separate package we get the benefit of:

1) Enforcing 3rd-party code using ttorrent as a library has the same privileges as command-line entry-points.

2) Uncluttering some key classes making them easier to scan and comprehend.

The moved method bodies and javadoc are kept as identical as possible, with only two non-move changes to make it possible: (a) Adding the Tracker#getTrackedTorrents() getter, and (b) making Client.ClientShutdown public.