Expose marker alpha option

[cammace]

Looks like we give the option to set the marker alpha when creating a view adapter but don't expose the API in MarkerViewOptions().

Should look something like this:

MarkerViewOptions marker = new MarkerViewOptions()
                        .position(new LatLng(-33.85699436, 151.21510684))
                        .alpha(0.5f);

@tobrun @bleege @zugaldia @ivovandongen

[tobrun]

Great catch @cammace!

[tobrun]

This landed in 7942ce9 in the android-v4.1.0 release branch. Closing