Dispose leaked resources revealed by profiling

- Break references of ITextBuffer keeps to classifiers
- Cancel async runs and stop timer before disposing feature objects
Merge pull request #506 from dungpa/dispose-resources

Dispose leaked resources revealed by profiling
ResolveUnopenedNamespace tagger skeleton
add ResolveUnopenedNamespaceSmartTaggerProvider
add LanguageService.GetAllEntitiesInProjectAndReferencedAssemblies
show top 5 entities as actions in ResolveUnopenedNamespaceTagger
ResolveUnopenedNamespaceTagger shows real type suggestions