Simplifying generated XPath when there is only a single sibiling

Hi Lukas,

Thanks for this great library. 

I am using jOOX with HTML having heard about it here
http://stackoverflow.com/questions/4746299/generate-get-xpath-from-xml-node-java

When jOOX generates an XPath, it will add [1] even if there is only a single element. For example for a HTML document the generated XPath will start /html[1]/body[1] even though we only expect one html and one body element. This is mainly a problem for human readability. 

I have modified the code to add a check to omit the sibling specifier if there is only a single sibling, and I have updated the unit tests accordingly.

What do you think?