Update to using Rackspace.Threading library instead of duplicating functionality
Add additional library targeting .NET 4.5
Resolve additional documentation links

* Add link to Rackspace.Threading library in Additional Reference Documentation project
* Fix two incorrect references in documentation comment and conceptual topic file
Use DelayedTask.WhenAll to simplify waiting in tests
Simplify waiting operations using DelayedTask and TaskBlocks.While
Remove unnecessary ParallelExtensionsExtras code
Include System.Net.Http libraries in all projects
Initial work to convert library to HttpClient
Fix handling of location headers
Fix handling of the User-Agent header
Updated handling for the Content-Type header in requests
Updated integration tests to work with the new HttpClient-based implementation
Increase test timeout to mitigate some timeout failures
Remove and sort usings
Base support for HttpResponseMessage validation
Customized validation for the QueueExistsAsync message (where 404 is a valid response)
Updates to address XML documentation build warnings in .NET 4.5 build
Updates to address XML documentation build warnings in .NET 4.0 build
checkpoint
Include files in all projects, and use proprocessor to eliminate code from the build