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
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Remove outdated information from troubleshooting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
Use os.path.realpath() before os.path.abspath()

Closes #454
Fixes #413
Make virtcol_current_gradient background color match line_current color

Fixes #449
Format in PowerlineLogger._log, not in pl.exception arguments
Use tuple in place of single-element list
Allow more then one -R be effective
Fix renderer_arg metavar
Add support for zsh local themes

Still missing: parser state

Ref #771
Make PS2 display something more meaningful: add continuation segment