Fixed leak while merging facets. The ThemeList that is used by OutputHelper to sort the Facets for an Output now uses a list to store the ManagedFacets. The correct order is made up by sorting the List using Collections.sort() function of the Java JDK. Therfore, the ManagedFacet class implements the Comparable interface. The return value of its compareTo(other) method depends on the value of the 'position' field.

- Validation of the input values of an incoming feed() call