In fish also check for presence of powerline in $PATH
Check for TMUX only once, check whether $TMUX is writeable

Fixes #801
Disable mode support on <=zsh-4.3.10

Fixes #800
Some style fixes
Move i3bgbar.py to i3bar.py
Merge branch 'i3-fixes' into develop
Merge pull request #806 from ZyX-I/shell-fixes

Shell fixes: TMUX support for su[do] and modes on old zsh