Added VaadinHttpService

Vaadin Http Service
Allow access to HttpServletRequest / HttpServletResponse

This can be used for example to set the REMEMBERME Token.

Example Usage:
```java
@Autowired
HttpService httpService;
...
HttpServletRequest request = httpService.getCurrentRequest();
request.setAttribute(AbstractRememberMeServices.DEFAULT_PARAMETER, rememberMe);
```

Original code from: https://github.com/markoradinovic/Vaadin4Spring-MVP-Sample-SpringSecurity
Original Author 'Marko Radinovic' included in JavaDoc
Shouldn't this be wrapped in  try... finally block?