Added terminus font and urxvt documentation.

How to use terminus and urxvt and still get the powerline glyphs .
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
Make powerline.vim work in compatible mode
More improved troubleshooting
Remove outdated information from troubleshooting
Add more spaces so that python indentation will be correct
A few style fixes
Remove duplicate troubleshooting question
Merge pull request #807 from ZyX-I/improved-troubleshooting

Improved troubleshooting
Use os.path.realpath() before os.path.abspath()

Closes #454
Fixes #413
Merge branch 'realpath' into develop
Make virtcol_current_gradient background color match line_current color

Fixes #449
Merge branch 'fix-449' into develop
Format in PowerlineLogger._log, not in pl.exception arguments