Path helpers fix (solves issue #323)

Hi, I'm unable to draw multipolylines or multipolygons with more or less than 2 polylines or polygons.
Look at this fiddle: http://jsfiddle.net/albertojacini/f8Y69/

The reason is that there's a constraint of 2 items in the validation function of these types. I guess it's a not wanted feat from a paste from the rectangle type... I removed them in these pull request and updated the examples with more than 2 shapes. They would otherwise throw this error as indicated by @jasonshark in the issue #323:
```
[AngularJS - Leaflet] Invalid data passed to the polyline path angular.js:9383
TypeError: Cannot call method 'on' of undefined
```
I hope this helps!