Updated WordPress readme for upcoming v1.3.0 of PSS.

- Removed some text.
- Incremented stable tag.
- Other teeny modifications.
Updated WordPress readme changelog.
Minified CodeMirror CSS files (unlikely to change anyways).
Increment version in pugin file & WP readme updates.

- Added notes about development moving to GitHub.
- Incremented version number in plugin file to 1.3.0.