DATAREST-221 - Added support for projections.

This commit introduces support to access resources via projections, which means naming a dedicated set of properties of the entity to be exposed and being able to refer to that set through a request parameter.

## General usage

Projections are defined as interfaces that mimic the properties of the domain class to be exported:

@Projection(types = Customer.class, name = "summary")
interface Summary {
  String getFirstname();
  String getLastname();
  AddressSummary getAddress();
}

interface AddressSummary() {
  String getZipCode();
}

The projection interface can be annotated with @Projection to be auto-discovered. We scan all packages in which we find domain types to be exported for projection types and auto-register them. For manual registration, use RepositoryRestConfiguration.projectionDefinitionConfiguration().addProjection(…) and manually register them.

If a projection is registered for a given type, this will be indicated via a "projection" template variable in the URI pointing to resources with projections. The name of the variable can also be configured on ProjectionDefinitionConfiguration.

## Internals

The projection interfaces are consider bean property delegates by default. This means, that for the above interfaces we will lookup the firstname, lastname and address property of the projection target. In the case of address we re-project the result of the proxy target invocation with a sub-projection onto AddressSummary.

For more advanced use-cases you can annotate a method of the projection interface with @Value and use a SpEL expression to invoke further functionality and return that to be rendered:

interface MyProjection {

  @Value("#{@myBean.someMethod(target)}")
  SubProjection getValue();
}

This projection would call the someMethod(…) method on a Spring bean named myBean handing the proxy target to the method. The result will be projected in turn onto a type called SubProjection.

As the projection objects are exposed to Jackson as is, they can be annotated with Jackson annotations to further customize the representation.