Added backslash to add_editor_style

Editor style css wasn't being applied for me until adding the backslash.
Use relative paths in README and docs TOC

https://github.com/blog/1395-relative-links-in-markup-files
Generate new roots.pot with makepot.php

See http://codex.wordpress.org/I18n_for_WordPress_Developers#Generating_a_POT_file

See #659