Don’t mess with hl()+contents

What does hl() mean? It looks like “highlight an empty string”, but is actually
“reset highlight style”. In addition if you are writing “hl()” function for new
renderer you need to care about “contents” variable (in two places!: None for
super() call and (contents or u'') after super() call) though function is
actually doing nothing with it. It is just inconvenient.
Use escape_hl_* variables for prompts deriving from ShellRenderer

This is faster then super() calls and also more convenient.
Fixes #142 just as well
Conflicts:
	powerline/renderers/bash_prompt.py
	powerline/renderers/ipython.py
	powerline/renderers/shell.py
	powerline/renderers/zsh_prompt.py
Clear highlight groups on ColorScheme event

Closes #196
Fixes #153
Fixes #194
Fix location of powerline.vim file

rm is uncommented because there is no “source_plugin” now.
Add more USE flags to gentoo ebuild
Add auto_rewrite method to pre_0_11

It raises error without this.
Move docs to readthedocs.org