Use 'span' element to display icons instead of 'i' element.

This patch replaces the ```<i>``` elements that are used to display the icons with ```<span>``` elements.

Scanned the sources for the following (without quotes):

- ```"<i "``` (ends with space)
- ```"</i>"```
- ```"'i'"```

If there are more, I'd be glad to know.

Potential users: Please read why this patch was written: http://wordpress.org/support/topic/question-about-the-use-of-the-i-element-to-display-icons