Fixes issue with stats on start event

In situations where a client is called like `docker stats` with no
arguments or flags, if a container which was already created but not
started yet is then subsequently started it will not be added to the
stats list as expected.

Also splits some of the stats helpers to a separate file from the stats
CLI which is already quite long.

Signed-off-by: Brian Goff <cpuguy83@gmail.com>