New Tracer spi

A new tracer spi is provided, and Apollo will now use Java ServiceLoader to load the specific service implementations.

Also a default implementation is provided, which will use CAT to do the trace job if cat-client is found in the class path, or a dummy implementation 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 and providing a service definition file 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 and ConfigPublishListener. A detailed developer documentation on this topic should be released soon (I hope...). @lepdou