Modified .gitignore - for support of all VS versions
Added benchmark for System.Threading.Tasks.Dataflow.BufferBlock
AsyncCollection: got rid of the TItemQueue type parameter
Added an old version of AsyncCollection<TItem, TItemQueue> for backward compatibility (just in case; is considered obsolete and will be removed eventually).
Fixed AsyncCollection's initialization if a non-empty item queue has been passed to the constructor
Added missing summary comments
Merge branch 'async-collection-ctor'
Added PriorityQueue, ConcurrentPriorityQueue and AsyncPriorityQueue classes