New windows in tmux sessions should open in $PWD

tmux 1.9 has changed the behaviour of new windows so they don't
automatically inherit the working directory of the current pane.

This change rebinds the c key to create a new window in the current
pane's PWD.

See http://sourceforge.net/projects/tmux/files/tmux/tmux-1.9/ (under the
"Incompatible Changes" heading) for more details.