fix for #60

[jcyuan]

1, for retrieve

segment.a / .b
triangle.a / .b / .c

because there is no such API on Collider
so implemented on vertices
if wanna retrieve segment.a / .b or triangle.a / .b / .c, use c.vertices instead

2, c.roundRadius() now available for all round shapes

tested and now c.borderRadius() and c.vertices() work correctly.

[sebcrozet]

Thanks!