Refactor _load_tasks()

This is to fix issue #20. TaskWarrior and TaskWarriorExperimental now have their own versions of _load_tasks().

TaskWarriorExperimental should continue to work in the same way *except* that searching by description has slightly changed due to a bug in TW (http://taskwarrior.org/issues/854). If you pass a description with parentheses (e.g. "(bw)") you'll get an error back from TW. I'm opening a pull request in Bugwarrior to deal with that problem.