add augroup indent_guides

Add augroup indent_guides, so that I can disable them by: autocmd! indent_guides VimEnter,Colorscheme * in the vimrc.local.
The reason why customers need it is that when someone using another light colorscheme, this setting is disharmony. Like this:
![aaaaa](https://cloud.githubusercontent.com/assets/3501200/4437776/4c83964a-479f-11e4-8755-971e61ec4f83.png)
When I disable the setting, the vim-indent-guides plugin automatically calculates
 the highlight colors. I get this which is much better:
![bbbbb](https://cloud.githubusercontent.com/assets/3501200/4437808/3827da0c-47a0-11e4-86b9-5f17eb620b3c.png)

Thanks for your sharing, it's great.