Consistency in export URLs

As described by http://platform.xwiki.org/xwiki/bin/view/Features/Exports , the export URLs are sensible to the format. Each format has a different use for the variable "pages" that describes the targeted pages but which is present in all the export URLs.

This variable should be as flexible as possible, as tried by the HTML format version. It should allow either regexp-like expresions or there should be varibles for specifying entire spaces.

Also, the variables present in the PDF format export URL like: includechilds (includechildren), includelinks, comments, attachments should be present in all format URLs.