fix Facebook share not working #24 issue

This should fix #24 issue if facebook app setup correctly
@gorbin, We will need to change gradle-wrapper.properties, if google update something, and build won't work without updating gradle version. Library should contains wrapper, so that it can be build without having standalone gradle installed. Plus note, that if someone wanted to build api_demos without launching Android Studio, for example from command line, with gradlew it will be very easy. Please remove "gradle", and "gradlew" from .gitignore, if we update gradle version, we will need to push this changes :)