Added missing static keywords for PHP Strict Standards

Fixes issues like:
```[13-Nov-2014 21:17:18 UTC] PHP Strict Standards:
call_user_func_array() expects parameter 1 to be a valid callback, non-static method
MathJax::mathjax_settings_link() should not be called statically in
/.../src/wp-includes/plugin.php on line 214
```