* Let users specify tasks directly under a Locust class, just as one would do it on a TaskSet class. These tasks will get the Locust instance as argument when executed.
* Removed Locust.task_set from the public API, and instead let users use either the tasks attribute or the @task decorator.
* Introduce a Locust.abstract boolean attribute. If it's set to True the Locust class is meant to be used as a base class, and users of that type can not be spawned directly from it.
As discussed in #83, removed HttpSession instantiation on the Locust.client attribute from base Locust class, and introduced new HttpLocust class with this functionality.
Added warning when trying to access the client attribute on a "bare" Locust instance. Updated documentation.