Multiple projects can be drawn on the same view

This patch allows multiple projects to be rendered to the same view.

The first project is draw first. The last added project is drawn last. [Here](http://opslag.bennyjacobs.nl/paper/my-examples/multiple_projects.html) is a example which uses this feature, different overlapping circles from different projects are visible in the same view.

Related issue: #295

Full source code from the example above:
```html
<!DOCTYPE html>
<html>
  <head>
    <title>multiple-projects-view</title>
    <style type="text/css">
        body {
	        margin: 0;
	        overflow: hidden;
        }
        #myCanvas {cursor: none;}
    </style>
    <script src="../src/load.js"></script>
  </head>
  <body>
  
    <canvas id="myCanvas" resize oncontextmenu="return false;"></myCanvas>
    
    <script>
		var canvas = document.getElementById('myCanvas');
		paper.setup(canvas);
		
		bottom_prj = paper.project;
		middle_prj = new paper.Project(paper.view);
		top_prj = new paper.Project(paper.view);
		
		bottom_prj.activate();
		var path = new paper.Path.Circle(new paper.Point(80, 50), 30);
		path.strokeColor = 'black';
		path.fillColor = new paper.Color(1, 0, 0, 0.6);
		
		middle_prj.activate();
		var path = new paper.Path.Circle(new paper.Point(90, 40), 30);
		path.strokeColor = 'black';
		path.fillColor = new paper.Color(0, 1, 0, 0.6);
		
		top_prj.activate();
		var path = new paper.Path.Circle(new paper.Point(100, 50), 30);
		path.strokeColor = 'black';
		path.fillColor = new paper.Color(0, 0, 1, 0.6);
		
		var cursor = new paper.Path.Circle(new paper.Point(100, 50), 5);
		cursor.strokeColor = 'black';
		cursor.fillColor = new paper.Color(1, 1, 0.5, 0.6);
		var tool = new paper.Tool();
		tool.onMouseMove = function(event) {
			cursor.position = event.point;
		}
		
		paper.view.draw();
		
    </script>
    
  </body>
</html>
```