add detector for unknown nullness of method argument.

currently 'testAnnotatedMethod()' test case cannot pass,
I am afraid that this is a bug of FindBugs.
detect unknown nullness of returned value.
make metadata and README.
call private method to avoid a bug of FindBugs.
performance enhancement.
improve method name.
Merge pull request #8 from eller86/unknown-nullness

Detect unknown nullness