Code review comment for lp:~thekorn/launchpad/make_iperson_ihasbugs

This is the first step to fix bug 282178 (making Person.searchTasks available over the webservice API)

This branch includes three different types of changes:
 * implemented Person.searchTasks() in a way that it can be called without BugTaskSearchParams and only by giving keyword arguments. Person.searchTasks() returns the same list of tasks which is shown as "related Bugs" in bugs.launchpad.net/~username.
 * as a person cannot have official_bug_tags this field is moved away from IHasBugs - thanks to Gavin Panella for helping me with the related changes.
 * added testcases for the internal API and the webservice API

The seconds step to fix the bug mentioned above will be to add (alias like) methods to IPerson (searchCommentedTasks(), searchSubscribedTasks() ...)