Changes

Extract MechJebLib as separate project/dll

MechJebLib.dll has no references to MJ/Unity/KSP (pure C#)
MechJebLibBindings.dll has references to Unity/KSP (conversion glue)
alglib.dll is extracted so that MJLib+MJ can both reference it
Jetbrains.Annotations is included so MJLib can use it without
  referencing Unity and MJ now uses the updated nuget package as well
  (Unity version is old and now will never get updated)

The TARGET_DIR now contains:

```
JetBrains.Annotations.dll
JetBrains.Annotations.xml
MechJeb2.dll
MechJeb2.pdb
MechJebLib.dll
MechJebLib.pdb
MechJebLibBindings.dll
MechJebLibBindings.pdb
System.Core.dll
System.dll
alglib.dll
alglib.pdb
mscorlib.dll
```

System.Core.dll, System.dll, mscorlib.dll shouldn't ship with the zip
file, but everything else should go into the MechJeb/Plugins directory

System.Buffers.dll can be removed it hasn't been necessary for quite
some time.

Signed-off-by: Lamont Granquist <lamont@scriptkiddie.org>
 (commit: 7d329fd)