Add the status_change (date) to the PackageListing json output
Fix the unit-tests for the addition of the status_change date
Bug fix in the api packages

- Added an ``acls`` argument to return the ACLs information with the results
- Fix documentation
- Fix search packages in multiple branches
- Do not return the package information twice
- Return only the ACL concerning the branches of interest if the call was restricted to some branches.
Small css fix: allow link in the accordion widget (useful in the API documentation)
More personalisation of JSON returned

Using package=True/False one can specify if the PackageListing json representation
should also include a representation of the package object it's linked to.
This allows reducing the amount of duplicated information in the JSON output of some API calls.

- Using collection=['<branchname>'] one may restrict the representation of
the ACLs in JSON to only the specified branches, thus omitting the other branches.
Make the package object hashable (which allows to use them in a set)
Use set() when searching packages to avoid sending duplicates