Add a working 3.5 version of the TaskScheduler using the downport of the TPL made by microsoft

Conflicts:
	src/Net45/NetMQ.Net45.csproj
	src/Net45/Properties/AssemblyInfo.cs
Implement .NET 3.5 compatibility changes: Move all .NET 4+ specific code to a seperate library, and make the main NetMQ lib a .NET 3. lib

Conflicts:
	src/Net45/Properties/AssemblyInfo.cs
Add 3.5 project to the solution, fix a messup in previous commit with the AssemblyInfo.cs of the 4.5 project