replace if null with existence operator, closes #13

Working with assignment and return values
Analyzer to check if disposable types are calling GC.SuppressFinalize method.
Merge pull request #89 from giggio/ExistenceOperator

Replace if null with existence operator
Merge pull request #92 from israelaece/SuppressFinalize

Analyzer to check if disposable types are calling GC.SuppressFinalize