use new temp directory for npm

download and unzip npm in a temp directory under the nvm root. fix issue
#46

src/nvm.go

@@ -211,20 +211,24 @@ func install(version string, cpuarch string) {
 
     // If successful, add npm
     npmv := getNpmVersion(version)
-    success := web.GetNpm(getNpmVersion(version))
+    success := web.GetNpm(env.root, getNpmVersion(version))
     if success {
       fmt.Printf("Installing npm v"+npmv+"...")
 
+      // new temp directory under the nvm root
+      tempDir := env.root + "\\temp"
+
       // Extract npm to the temp directory
-      file.Unzip(os.TempDir()+"\\npm-v"+npmv+".zip",os.TempDir()+"\\nvm-npm")
+      file.Unzip(tempDir+"\\npm-v"+npmv+".zip",tempDir+"\\nvm-npm")
 
       // Copy the npm and npm.cmd files to the installation directory
-      os.Rename(os.TempDir()+"\\nvm-npm\\npm-"+npmv+"\\bin\\npm",env.root+"\\v"+version+"\\npm")
-      os.Rename(os.TempDir()+"\\nvm-npm\\npm-"+npmv+"\\bin\\npm.cmd",env.root+"\\v"+version+"\\npm.cmd")
-      os.Rename(os.TempDir()+"\\nvm-npm\\npm-"+npmv,env.root+"\\v"+version+"\\node_modules\\npm")
+      os.Rename(tempDir+"\\nvm-npm\\npm-"+npmv+"\\bin\\npm",env.root+"\\v"+version+"\\npm")
+      os.Rename(tempDir+"\\nvm-npm\\npm-"+npmv+"\\bin\\npm.cmd",env.root+"\\v"+version+"\\npm.cmd")
+      os.Rename(tempDir+"\\nvm-npm\\npm-"+npmv,env.root+"\\v"+version+"\\node_modules\\npm")
 
-      // Remove the source file
-      os.RemoveAll(os.TempDir()+"\\nvm-npm")
+      // Remove the temp directory
+      // may consider keep the temp files here
+      os.RemoveAll(tempDir)
 
       fmt.Println("\n\nInstallation complete. If you want to use this version, type\n\nnvm use "+version)
     } else {

src/nvm/web/web.go

@@ -10,6 +10,7 @@ import(
   "strings"
   "strconv"
   "../arch"
+  "../file"
 )
 
 var client = &http.Client{}
@@ -81,9 +82,21 @@ func GetNodeJS(root string, v string, a string) bool {
 
 }
 
-func GetNpm(v string) bool {
+func GetNpm(root string, v string) bool {
   url := "https://github.com/npm/npm/archive/v"+v+".zip"
-  fileName := os.TempDir()+"\\"+"npm-v"+v+".zip"
+  // temp directory to download the .zip file
+  tempDir := root+"\\temp"
+
+  // if the temp directory doesn't exist, create it
+  if (!file.Exists(tempDir)) {
+    fmt.Println("Creating "+tempDir+"\n")
+    err := os.Mkdir(tempDir, os.ModePerm)
+    if err != nil {
+      fmt.Println(err)
+      os.Exit(1)
+    }
+  }
+  fileName := tempDir+"\\"+"npm-v"+v+".zip"
 
   fmt.Printf("Downloading npm version "+v+"... ")
   if Download(url,fileName) {