Make ConsulConfigurationSource a public class

[Shuvajit Das]

ConsulConfigurationSource is package private but it is the return value of the public function, ConsulConfigurationSourceBuilder::build. This causes problems when using kotlin b/c kotlin cannot assign any variable to type ConsulConfigurationSource since it is package private. Since ConsulConfigurationSourceBuilder::build returns an instance of ConsulConfigurationSource, we should make ConsulConfigurationSource a public class.

[Shuvajit Das]

Fixed by #198

[Norbert Potocki]

Thanks for addressing this!