Linux build failure due to case sensitivity & nunit-console wrong param value spelling

Addresses the following 2 issues:
1). Build fails in linux + mono (or other systems that are case sensitive) due to wrong case for NuGet.Core.dll dependency in FakeLib
Specifically: in src/app/FakeLib/FakeLib.fsproj, NuGet.Core is referenced by path: ..\..\..\packages\NuGet.Core.2.8.1\lib\net40-Client\NuGet.Core.dll which is unresolvable in case sensitive system since the proper path on the file system is: "..\..\..\packages\Nuget.Core.2.8.1\lib\net40-Client\NuGet.Core.dll"
This causes the dependency to become unresolved and the xbuild fails

2). Wrong value for /process argument is passed to nunit-console due to spelling error
Specifically: in src/app/FakeLib/UnitTests/NUnit/Common.fs, NUnitProcessModel.SeperateProcessModel, proper spelling is Separate, and in NUnitProcessModel.ParamString "Seperate" is give as the parameter value whereas the /process parameter accepts "Separate" (see: http://www.nunit.org/index.php?p=consoleCommandLine&r=2.6.3)