IMP Allow xml as str in lists for building
Add a function to clean the xml string

This is added because the other parts of the xml are generated
without formating.
Added 2 tests more

* Check the new feature to add an xmlstring directly
* Check the function to clean an XML
Add the example of the new feature (XML String)
Merge pull request #32 from totaler/allow_xml_str

IMP Allow xml as str in lists for building