Set default TMUX_SESSION_NAME to epoch time

Updated `TMUX_SESSION_NAME` to set to epoch time for the default tmux
session name.  This allows for a unique name, if not set by the user
via command line.  You can still use `-ns` and `-ts` as well, it just makes it to
where it does not break if you create a new tmux-cssh session in
another window in your terminal.

Without this, if you run `tmux-cssh` without adding `-ns` or `-ts [name]` specifically (in a different window/tab in your terminal), it (by default) connects you to the already existing session and you never connect to a different set of servers.

Example:

    tmux-cssh -u username server0{1,2}  ### Connects as tmux-cssh

...10 Minutes later a new tab is opened in the terminal application

    tmux-cssh -u username server0{3,4}  ### Connects as tmux-cssh, also, and shows you server0{1,2}; never connects to 03/04

Again - you can bypass this with `-ns` and `-ts [name]`, but this patch basically keeps it from "breaking".