Some changes for cleaning the sections after leaving a page.

Sry, the changes at the README.md are out of scope. But I missed it to change the readme and add the 'default' attribute to the sample code.  

Also i run in trouble by using spapp with many objects in the sections. After leaving a section some times the application slows down. I solved this by clearing the sections content after leaving. Sections without a 'src' attribute will be ignored.