Google Maps API v2 Upgrade

This PR fixes #50 upgrading to Google Maps Android API v2. It does clustered markers, exploding as you get closer in. Clusters consisting of all members in the same place behave like a dark-colored icon on the website (with more than one host in the popup).

A release APK for testing and experimentation is at https://dl.dropboxusercontent.com/u/7350603/Warmshowers/wsandroid_v2_api/wsandroid-release.apk