Added JAXB Codec

Added module for encoding and decoding using JAXB.
Thanks for the feedback Adrian.  What do you think about this?  I added a constructor to the module for specifying the factory and then I create the factory with a builder that allows you to set those jaxb options if you feel inclined.

```java
@dagger.Module(injects = Feign.class, addsTo = Feign.Defaults.class)
public final class JAXBModule {
    private final JAXBContextFactory jaxbContextFactory;

    public JAXBModule() {
        this.jaxbContextFactory = defaultJAXBContextFactory();
    }

    public JAXBModule(JAXBContextFactory jaxbContextFactory) {
        this.jaxbContextFactory = jaxbContextFactory;
    }

    @Provides
    public Encoder encoder(JAXBContextFactory jaxbContextFactory) {
        return new JAXBEncoder(jaxbContextFactory);
    }

    @Provides
    public Decoder decoder(JAXBContextFactory jaxbContextFactory) {
        return new JAXBDecoder(jaxbContextFactory);
    }

    @Provides
    @Singleton
    JAXBContextFactory defaultJAXBContextFactory() {
        return new JAXBContextFactory.Builder().build();
    }
}
```