Detect .NET 4.5 More inteligently

[damageboy]

@roji wrote, in #3
You're doing conditional injection of these for .NET 4.5 only, which is good, but you require users to pass --platform in order to know which .NET we're on. Deduce this from the DLL you're injecting instead...

This can be done with the following asm level attribute:

[assembly: TargetFramework(".NETFramework,Version=v4.5", FrameworkDisplayName = ".NET Framework 4.5")]

Instead of the --target flag