Junit rule to instantiate default Jadler instance more easily

Jadler is already pretty easy to use. Although not a new feature, Junit rules become more and more popular when setting up a test compared to the traditional setup/teardown pattern as the often require less code. 

This change set is a simple Junit Rule which instantiates Jadler via an @Rule annotation. By this the setup of Jadler becomes even easier as no special setup method is required anymore.
```
    @Rule
    public JadlerRule defaultJadler = new JadlerRule();

```
I found it quite useful and was using it for quite a while with a private extension. Would be nice if you consider this useful as well and integrate into master. If you any questions and need changes before integration just let me know.