CurvedArrow Interface

The interface of class `CurvedArrow` in [geometry.py](https://github.com/3b1b/manim/blob/465951ab88211e4a07e1a17b7ad370cb662f7d06/mobject/geometry.py#L141) is not trivial to understand.

An example is in the [tutorial](https://talkingphysics.wordpress.com/2018/06/12/more-shapes-manim-series-part-3/), basically:
```python3
from big_ol_pile_of_manim_imports import *

class MoreShapes(Scene):
# A few more simple shapes

def construct(self):
pointer = CurvedArrow(start_point=5 * RIGHT, end_point=2 * RIGHT, color=MAROON_C)

self.add(pointer)
```

The output animation will have a curved arrow pointing from the `end_point` to the `start_point`. But intuitively, it should point from the `start_point` to the `end_point`. (The link above to the tutorial have an embedded video showing the curve.)