Handle wrapping of network streams, and auto download nuget

[warrenfalk]

Great work on an awesome project, here. Thanks.

Two things in this pull request

1. nuget.exe is not on the path by default always in Windows, so paket.cmd fails. I added code that detects if this is true and auto downloads it so that it is really possible to clone and build.
2. The decoding stream fails if you wrap a network stream because it expects every call to Read() to read every byte it asks for, but network streams often return short reads. I ran into this trying to decode a file as it was being downloaded from S3, but it failed every time unless I downloaded the whole file to a memory stream first, and then wrapped the memory stream. So this change adds a test that simulates the shortest possible reads (which fails) and adds the fix for it.

[MiloszKrajewski]

Thank you.
I had the same bug in old lz4net but forgot about it. Shame on me.
On top of this I made a little bit of a mess recently removing dictionary id from header so I will fix those two things at the same time ASAP.
Thanks again.