JSR-305 Support

AutoValue have @Nullable support which is great. But what about other JSR-305 capabilities.
For example, I could define annotation like
```
@Documented
@Nullable
@TypeQualifierDefault(ElementType.METHOD)
@Retention(RetentionPolicy.SOURCE)
public @interface MethodsReturnNullableByDefault {}
```
To remove repetitive @Nullable annotations for POJO's
This annotation will work perfectly with many tools which have JSR-305 support but AutoValue will still generate guard checks for nulls which will behave inconsistently with lint/IDE checks.