Expose Traverson default message converters in a convenience method

The existing encapsulation of RestTemplate (via #201, #203) makes it the 
user's responibility to set up the correct message converters (which as argued
in #203, is necessary), but it doesn't give any help knowing what the
converters should be.

This change exposes the default message converters via a static convenience
method, e.g.

    RestTemplate restTemplate = new RestTemplate();
   restTemplate.setInterceptors(Arrays.<ClientHttpRequestInterceptor>
asList(interceptor));
  
restTemplate.setMessageConverters(Traverson.getDefaultMessageConverters(MediaTypes.HAL_JSON));