Downgrade Guava from 14 to 13 to address the uncomfirmed issue #34

Refer to https://github.com/johncarl81/transfuse/issues/34

All the changes are replacing FluentIterable.toSet() to .toImmutableSet(). All the changes except one should be correct as the return type is an ImmutableSet anyway, and the exception case is at AnnotationProcessorBase.java that the return type is a plain Set that my change make the return Set immutable. ~~This doesn't cause any error in my app, however.~~ (The tested version used Sets.newHashSet() instead)

For review:
```
public Set<String> getSupportedAnnotationTypes() {
        Class<? extends Annotation>[] supportedAnnotations = getClass().getAnnotation(SupportedAnnotations.class).value();

        return FluentIterable
                .from(Arrays.asList(supportedAnnotations))
                .transform(new ClassToNameTransform())
                .toImmutableSet();//changed from .toSet()
    }
```