Improve support of sub/superscript

![sub-super-strikethrough](https://cloud.githubusercontent.com/assets/1052079/3809642/8064f344-1c86-11e4-9597-eea9439d0b3d.png)


Test document: https://drive.google.com/file/d/0B1EH5OPb-RrjbW9DQ1VRLUxQUEE/edit?usp=sharing
I could not find a definition of what "white space separated" exactly means in "The style:text-position attribute has one or two white space separated values.". So for now I assume in my superficial knowledge it could mean that first better collapsing is to be done on the whitespaces in the list, if it is to be a list value as specified by XML (cmp 2.5.1.2 List datatypes, where the constraining facets contains "whiteSpace" with a fixed value of "collapse". http://www.w3.org/TR/xmlschema-2/#atomic-vs-list).