Update diagnostic categories to exactly match StyleCop documentation
Merge pull request #297 from sharwell/categories

Update diagnostic categories to exactly match StyleCop documentation
Implement diagnostic SA1200 UsingDirectivesMustBePlacedWithinNamespace (fixes #60)
Implement diagnostic SA1208 SystemUsingDirectivesMustBePlacedBeforeOtherUsingDirectives (fixes #68)
Implement diagnostic SA1209 UsingAliasDirectivesMustBePlacedAfterOtherUsingDirectives (fixes #69)
Implement diagnostic SA1210 UsingDirectivesMustBeOrderedAlphabeticallyByNamespace (fixes #70)
Implement diagnostic SA1211 UsingAliasDirectivesMustBeOrderedAlphabeticallyByAliasName (fixes #71)