Corresponding to Google Maps SDK 1.6.1

Hello, 

I maintained Google Maps SDK binding to SDK 1.6.1, so please pull it.

Some thigs to be consideration:

1. Constant shown below is newly added, but I can't modify libGoogleMapsExporter.a, so not care adout this.  
I think libGoogleMapsExporter.a should also be open source.  
  
*GMSMarkerLayer.h*  
```cs

extern NSString *const kGMSMarkerLayerOpacity;

``` 

2. GMSGeometryUtils.h contains many useful C static functions, but I can't port it, so not care about them.

3. GMSMarker and GMSTileOverlay have new feature property "opacity", but base class CALayer also has
opacity property, so warnings are shown in compiling phase. 
So I port them as "MarkerOpacity" or "TileOpacity".