Add windows compilation instructions to the README.

For our command line shy friends on windows.
Don't use Action<> in DllImport uv_fs_read.

The .NET runtime doesn't want to marshal it.
Specify delegate marshalling policy.

.NET has problems with this as well.

Don't know yet if I have to save the delegate. As far as I remember
using StaticEnd like that would create a delegate on the fly which could
go out of scope and because of GC and the resulting pointer would be
null.
Shorten code for Process Fixture.

No need to repeat the different same commands for just two different
settings.
use xunit testing framework.

NUnit just throws errors, is hard to set up.
Just use something that works nicely.

xUnit is the succcessor of nunit anyway.