[FIX] point_of_sale: show_popup('name','comment') should display the comment in the popup
[IMP] refactoring: move function topological_sort to openerp.tools
[FIX] models: default_get() used to miss values

The default values are computed by evaluating fields on a new record. The fix
retrieves values from the cache earlier, because in some cases, the evaluation
of a field invalidates a formerly evaluated field.
[FIX] base: tests: move test menu under correct root