Feature-detect .NET45 extensions on BinaryReader/BinaryWriter

[t0yv0]

How about this? With this commit, it only builds "lib/net40/FsPickler.dll" for .NET 4.0. The only functionality the project needs is extra constructors on BinaryReader/BinaryWriter with leaveOpen. With the latest commit, the following happens:

1. Detect if these constructors are defined at runtime, and if yes, use them;
2. If not available, use a shim that wraps Stream to ignore the .Close method

So the interface is the same, and unlike previous solution, there's only one assembly, and the new constructors get used on .NET 4.5.

I was not sure how to run tests, could you look into it? Running tests on .NET 4.0 especially would be important.

Other possible problems I can think of:

• in restricted contexts this may fail if code generation is not allowed (Linq Expression trees)

[t0yv0]

In the end I actually don't know which is better - so feel free to ignore this PR if you prefer #13.

Advantages of #14:

• Single assembly
• Smaller .nupkg size
• Always uses .NET 4.5 functionality if present, even in apps compiled against .NET 4.0

Advantages of #13:

• No reflection/codegen

[eiriktsarpalis]

Ok, I'll just go ahead and close this. I will update nuget as soon as I test this with .NET 4.0

[t0yv0]

Would be much appreciated!

As I suggest in the #14 thread, you might get away with testing on .NET 4.5 by "emulating" 4.0 - forcing the shims to run. Not as good as testing on .NET 4.0 but should be good enough, I don't expect problems here..