Fixed typo, fixed, warning, improved CSS

* Readme: fixed Wordpress into WordPress (capital P). 
* Readme: Removed reference to Ghost – dunno if that’s acceptable, but I thought Markdown with preview has existed before Ghost, so why cross-reference it in documentation?
* Fixed a warning thrown when a new post was yet unsaved and therefore `$_POST` was empty.
* Various improvements on UI in style.css.