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
Reintroduce admin formatting changes, Replace all variants of <br> tags in MathML code
Code formatting corrections
Prevent TinyMCE from striping MathML tags
Merge pull request #5 from jwenerd/master

Include Code style cleanup; sanitization; MathML tags and re-introduce Admin formatting changes