Update README for 1.3

* remove trailing spaces
* Tested up to --> 4.0.0
* describe changes in 1.3.0
Coding style changes + sanitization

* add whitespace per WordPress coding standards
* sanitize input on save
Coding style changes + sanitization

* bump version to 1.3.0
* add whitespace per WordPress coding standards
* make methods static
* Whitelist MathML tags
* remove <br/>s inside <math> blocks
use tabs instead of spaces