tmux: Set up split bindings according to tmux version.

In tmux 1.9 and above, my old split keybindings stopped working. Since
I'm using both 1.8 and 1.9 though, I was looking for a unified solution.
tmux: Remove socket detection since tmux does that already.

Tmux looks for the TMUX environment variable which is already set when
this file gets executed.
tmux: Snag some bindings from tmux-sensible plugin.
Merge branch 'rework_tmux'

This fixes a few of the troubles I'd been having with tmux. Especially
now I can split panes across tmux 1.8 and 1.9.