#207 allowing disabling watchers on markers

I created the attribute ```watchMarkers``` which will take a true/false in order to watch on markers properties or not.

When the watchers are disabled, if you change a marker attribute, it won't take any effect as expected. 

Besides that, the watch that remains on ```markers``` attribute was testing for object equality, which was also badly affecting performance for large amounts of data. So I'm also conditionally changing it to not test for equality when ```watchMarkers``` is set to false. Because of that you'll have to give a new markers object to update the map instead of deleting/adding new markers to the given object.

eg:
``` html
<leaflet markers="markers" watchMarkers="false"></leaflet>
```

Everything remains just the same if you don't specify anything.