added isosceles polygon shape:
- set number of pointes
- set rotation (degrees) clockwise
- 'iso-poly' with numPoint=3 equals 'triangle'. add 'rotate': 60 to turn it on its head
- 'iso-poly' with numPoint=4 equals 'diamond'. add 'rotate': 45 to make it a 'square'
renamed isosceles to equilateral, of course..
also updated online sample on https://dl.dropboxusercontent.com/u/45136256/sigma.js/nodeshapes.html