added nvm unload method to remove nvm from shell

nvm.sh

@@ -218,6 +218,7 @@ nvm() {
       echo "    nvm alias <name> <version>  Set an alias named <name> pointing to <version>"
       echo "    nvm unalias <name>          Deletes the alias named <name>"
       echo "    nvm copy-packages <version> Install global NPM packages contained in <version> to current version"
+      echo "    nvm unload                  Unload NVM from shell"
       echo
       echo "Example:"
       echo "    nvm install v0.10.24        Install a specific version number"
@@ -556,6 +557,10 @@ nvm() {
     "--version" )
         echo "nvm v0.3.0"
     ;;
+    "unload" )
+      unset -f nvm nvm_print_versions nvm_checksum nvm_ls_remote nvm_ls nvm_remote_version nvm_version nvm_rc_version > /dev/null 2>&1
+      unset RC_VERSION NVM_NODEJS_ORG_MIRROR NVM_DIR NVM_CD_FLAGS > /dev/null 2>&1
+    ;;
     * )
       nvm help
     ;;