PLOTCUBE - Display a 3D-cube in the current axes

PLOTCUBE(EDGES,ORIGIN,ALPHA,COLOR) displays a 3D-cube in the current axes
with the following properties:
* EDGES : 3-elements vector that defines the length of cube edges
* ORIGIN: 3-elements vector that defines the start point of the cube
* ALPHA : scalar that defines the transparency of the cube faces (from 0
to 1)
* COLOR : 3-elements vector that defines the faces color of the cube

Example:
>> plotcube([5 5 5],[ 2 2 2],.8,[1 0 0]);
>> plotcube([5 5 5],[10 10 10],.8,[0 1 0]);
>> plotcube([5 5 5],[20 20 20],.8,[0 0 1]);