New Tracer spi
A new [tracer](https://github.com/ctripcorp/apollo/blob/master/apollo-core/src/main/java/com/ctrip/framework/apollo/tracer/Tracer.java) spi is provided, and Apollo will now use Java ServiceLoader to load the specific service implementations.
Also a [default implementation](https://github.com/ctripcorp/apollo/blob/master/apollo-core/src/main/java/com/ctrip/framework/apollo/tracer/internals/DefaultMessageProducerManager.java) is provided, which will use [CAT](https://github.com/dianping/cat) to do the trace job if `cat-client` is found in the class path, or a [dummy implementation](https://github.com/ctripcorp/apollo/blob/master/apollo-core/src/main/java/com/ctrip/framework/apollo/tracer/internals/NullMessageProducerManager.java) will be used if `cat-client` is not found. Which also means `cat-client` is _**optional**_ now!
Users are encouraged to provide their own tracer implementations, by simply implementing the [spi interfaces](https://github.com/ctripcorp/apollo/tree/master/apollo-core/src/main/java/com/ctrip/framework/apollo/tracer/spi) and providing a [service definition file](https://github.com/ctripcorp/apollo/blob/master/apollo-core/src/main/resources/META-INF/services/com.ctrip.framework.apollo.tracer.spi.MessageProducerManager) in `classpath:META-INF/services`(the implementation and service definition file should be in a separate jar file in the classpath), and it should just work!
Email support when configurations are released
Now Apollo has the capability to send emails when configurations are released, in case other project members are interested. The email will contain the stuff displayed in release histories, such as operator, release time, configuration delta, etc.
For more information, please refer [EmailService](https://github.com/ctripcorp/apollo/blob/master/apollo-portal/src/main/java/com/ctrip/framework/apollo/portal/spi/EmailService.java) and [ConfigPublishListener](https://github.com/ctripcorp/apollo/blob/master/apollo-portal/src/main/java/com/ctrip/framework/apollo/portal/listener/ConfigPublishListener.java). A detailed developer documentation on this topic should be released soon (I hope...). lepdou