Add nvm_is_valid_version

nvm.sh

diff --git a/nvm.sh b/nvm.sh
index 7e1a961..497ad2b 100644
--- a/nvm.sh
+++ b/nvm.sh
@@ -263,6 +263,22 @@
   fi
 }
 
+nvm_is_valid_version() {
+  if nvm_validate_implicit_alias "$1" 2> /dev/null; then
+    return 0
+  fi
+  case "$1" in
+    "$(nvm_iojs_prefix)" | "$(nvm_node_prefix)")
+      return 0
+    ;;
+    *)
+      local VERSION
+      VERSION="$(nvm_strip_iojs_prefix "$1")"
+      nvm_version_greater "$VERSION"
+    ;;
+  esac
+}
+
 nvm_normalize_version() {
   echo "$1" | command sed -e 's/^v//' | command awk -F. '{ printf("%d%06d%06d\n", $1,$2,$3); }'
 }

test/fast/Unit tests/nvm_is_valid_version

diff --git a/test/fast/Unit tests/nvm_is_valid_version b/test/fast/Unit tests/nvm_is_valid_version
new file mode 100755
index 0000000..d34f767
--- /dev/null
+++ b/test/fast/Unit tests/nvm_is_valid_version
@@ -0,0 +1,13 @@
+#!/bin/sh
+
+die () { echo $@ ; exit 1; }
+
+. ../../../nvm.sh
+
+nvm_is_valid_version 0.1.2 || die "nvm_is_valid_version 0.1.2 did not return 0"
+nvm_is_valid_version foo && die "nvm_is_valid_version foo did not return 1"
+nvm_is_valid_version iojs-1 || die "nvm_is_valid_version iojs-1 did not return 0"
+nvm_is_valid_version iojs || die "nvm_is_valid_version iojs did not return 0"
+nvm_is_valid_version node || die "nvm_is_valid_version node did not return 0"
+nvm_is_valid_version stable || die "nvm_is_valid_version stable did not return 0"
+nvm_is_valid_version unstable || die "nvm_is_valid_version unstable did not return 0"