[Checkers] Checkers naming

[Michael Ernst]

I've been thinking about names, too.  I don't like Checkers Framework,
either, but had a more modest proposal of changing it to Checker Framework
(singular instead of plural).  I'm open to bigger changes, though.

Also, I would  like to propose changing the name of the NonNull checker.
(We could do this in the next release  as well, but that's not as critical
as choosing a new name.)  NonNull is a type qualifier, not a checker
name, and we don't expect people to even use that type qualifier in
general.  I like "null dereference checker", or NullDeref checker.  (An
alternative is "nullness checker", but that isn't as specific about what
the checker actually checks.)

Thoughts?

                    -Mike