Add installer (#1)

* Add installer

* Add tests

* install-dotnet script should be in externals

* merge

* Fix tests

* Fix naming

* Clean up

* Feedback

externals/get-os-platform.ps1

@@ -0,0 +1,18 @@
+function Get-Machine-Architecture()
+{
+    # possible values: AMD64, IA64, x86
+    return $ENV:PROCESSOR_ARCHITECTURE
+}
+
+function Get-CLIArchitecture-From-Architecture([string]$Architecture)
+{
+    switch ($Architecture.ToLower())
+    {
+        { ($_ -eq "amd64") -or ($_ -eq "x64") } { return "x64" }
+        { $_ -eq "x86" } { return "x86" }
+        default { throw "Architecture not supported. If you think this is a bug, please report it at https://github.com/dotnet/cli/issues" }
+    }
+}
+
+$CLIArchitecture = Get-CLIArchitecture-From-Architecture $(Get-Machine-Architecture)
+Write-Output "Primary:win-$CLIArchitecture"