Add @NotNullByDefault annotation

[Abnaxos]

It's badly needed. Since years, I have a strict NotNull policy in all of my code, so I simply annotate the whole package. Everyone knows this policy is intact, it's just about telling the IDE as well. If I annotate my NotNull code with NotNull, whenever NotNull is appropriate, my NotNull code basically consists of only NotNull with some occasional actual NotNull code between all the NotNull annotations from time to time. :)

Currently, I fix this by mixing with Eclipse's NonNullByDefault (unfortunately, Eclipse's NonNull and Nullable aren't applicable to methods, fields and parameters (Target({ElementType.TYPE_USE})), so I have to use JetBrains annotations for this).

The new annotation should be applicable to packages and classes, maybe also methods. I personally don't care about using it in types, but other people's mileage may vary. Specifying to what elements exactly it applies (methods, fields and/or parameters, maybe also type use) isn't strictly necessary for me, but probably makes sense. I'd suggest to default to all of them or no default at all.

This would also cover #18.

[J0s3f]

This would also be great in combination with the intellij-annotations-instrumenter-maven-plugin to have fields in specific packages always non-null and others not.