Added new CompositeHealthIndicator and updated EndpointAutoConfiguration to use it when multple DataSource beans are available.

Per [this Q&A](http://stackoverflow.com/questions/23449661/multiple-datasource-beans-with-spring-boot-actuators-endpointautoconfiguration/23450266#23450266) on stackoverflow with Dave Syer earlier today, I've signed the contributor license agreement and created a CompositeHealthIndicator (and respective test case).