Fixed KRuntime to build on mono/Linux

A Lot of changes were required to the build script, but most of the changes are just addition of if='IsMono' and if='!IsMono'. Highlights of some changes, have to add Linux directory to the nuspec folder, because the the current .nuspec files have file paths that follow windows file system structure. so i rewrite them and placed them  in the nuspec\Linux folder and directed build script to them if were are on Linux/mono and use the windows versions if we are on windows. Their is only one change in the source code which was a trivial one. I am here to answer any query about the changes i have a reason for every change.Personally tested it on both windows and Linux.
info:   program: .nuget/NuGet.exe
info:   commandline: pack "artifacts/build/KRE-mono45-x86.nuspec" -OutputDirectory "/home/shahid/AspVnext/KRuntime/artifacts/build" -Version 0.1-alpha-t140611183427 -NoPackageAnalysis
info:   workingdir: /home/shahid/AspVnext/KRuntime
Cannot open assembly '.nuget/NuGet.exe': No such file or directory.
warn: Exit code 2 from .nuget/NuGet.exe

Note:
when i change .nuget/NuGet to .nuget/nuget in the build.sh this happens, even after i delete cachedir and .nuget directory. actually this change is a necessity for the build to not break on Linux