If you select a brush and click on it once in one of the 2D views, the square handles at the corners will turn into circles, which can then be clicked and dragged to rotate the object. By default, it will rotate in 1° increments, but there is an option in preferences to lock it to 15° increments instead, which can then be toggled off by holding down alt (or shift, I don't remember). You can also rotate a selection more precisely by opening the transform dialogue with control + M. (Quick tip: sometimes, when rotating using the transform dialogue, your selection will rotate slightly on axis set to 0; to fix this, set those axis to 0.0 instead)