Add support for beans{} in CLI scripts

User can add (a single) beans{} DSL declaration (see GroovyBeanDefinitionReader
in Spring 4 for more detail) anywhere at the top level of an application source
file. It will be compiled to a closure and fed in to the application context
through a GroovyBeanDefinitionReader. Cool!

The example spring-boot-cli/samples/beans.groovy runs in an integration test
and passes (see SampleIntegrationTests).