add ratpack-spring-boot module

This PR adds a SpringBackedRegistry implementation and a simple helper for starting a Spring Boot application embedded in Ratpack. The ratpack.spring.Spring.run method returns a Registry instance that can be passed to the "register" DSL method. 
usage example: https://github.com/lhotari/ratpack-gorm-example/blob/dc4ebfafec36976cdc77924d1c9c9da9f6e9fda1/src/ratpack/Ratpack.groovy#L10

Based on discussion in Ratpack Forum: http://forum.ratpack.io/Preparation-of-Spring-Boot-module-for-Ratpack-tp569p587.html , I've now re-implemented SpringBackedRegistry by copying the InjectorBackedRegistry and "porting" that to use Spring and re-using some code from [the version of SpringBackedRegistry that is implemented by Dave Syer](https://github.com/dsyer/spring-boot-ratpack/blob/master/spring-boot-ratpack/src/main/java/ratpack/spring/internal/SpringBackedRegistry.java).
I added tests for SpringBackedRegistry by picking some test methods from the tests of the different Registry implementations that Ratpack already contains.