Fix hyperrectangle plot

[mforets]

for example, h = Hyperrectangle(center=[0.005, 1.1], radius=[0.005, 0.861785]) displays two triangles.

[schillic]

vertices_list returns the vertices neither in clockwise nor counter-clockwise fashion.

julia> h = Hyperrectangle([1., 2.], [3., 4.])
LazySets.Hyperrectangle{Float64}([1.0, 2.0], [3.0, 4.0])
julia> vertices_list(h)
4-element Array{Array{Float64,1},1}:
 [4.0, 6.0]  
 [-2.0, 6.0] 
 [4.0, -2.0] 
 [-2.0, -2.0]

I see two solutions:

1. Change the vertices_list implementation (also of all other AbstractPolytope types).
   If it makes the implementation more expensive, we should add a new function vertices_list_ccw and use this one for plotting. The function can be restricted to 2D sets if that helps.
2. Change the plot recipe such that it sorts the vertices. Algorithm: Start with any vertex, compute the angle to all other vertices, choose the smallest one, and draw the line.

[mforets]

good, i would try with your solution 2 relying on convex_hull.