add center option to scale and allow vector scale

--HG--
extra : amend_source : c217f376b0742ee73df2d9b64a3e6a1bb6063084
don't check for zero divide in scale
implement Shape.rotate

--HG--
extra : amend_source : 3ac47181e280183d8789de8136db893f4fe808d8
start_angle argument to Shape.regular_polygon now in radians
implement angular velocity
improve graphics_test
rename graphics_test to graphics_demo

--HG--
rename : tests/graphics_test.py => tests/graphics_demo.py
update readme
implement flip methods
setDataStyle to STYLE_NUMPY
test union and intersection
implement shape subtraction

--HG--
extra : amend_source : 2df05f190da47932e5dbe417645044104b1d57cb
don't throw error when comparing equality of Shapes with different number of vertices
test xor
addition operator does Shape union
there are always kwargs, no need for repr without them
put date in changelog
clarify flip docstring
Bump version: 0.1.2 → 0.2.0
Added tag v0.2.0 for changeset d1b40bf76b42