RestContext getters

This pull request is based on #283 but not only adds a getter for exception mappers but also for for marshallers.

The new methods introduced are:
```groovy
public ExceptionMapper getExceptionMapping(var $type)
public TypeMarshaller getMarshaller(var $type)
```

It also changes the `add` variants to return the added instance:
```groovy
public ExceptionMapper addExceptionMapping(var $type, ExceptionMapper $m)
public TypeMarshaller addMarshaller(var $type, TypeMarshaller $m)
```

Both of the above previously had a `void` return type.