New popwin key bindings `<SPC> w p m` and `<SPC> w p p`

To open messages buffer in a popup window and to close
current popup window
Add `C-k` and `C-j` to select auto-complete candidate
Add function spacemacs/remove-popwin-display-config

And add a tip to disable popwin for helm in readme
Update TOC of readme
Fix broken links in readme
Merge branch 'develop' (v0.3.1)