Do not encode local email parts

Punycode does not apply to local parts of email addresses, only to the domain name labels in it. Everything before `@` should be preserved.

Fixes #20 (comment).

README.md

@@ -100,7 +100,7 @@ punycode.toUnicode('xn----dqo34k.com');
 // → '☃-⌘.com'
 
 // decode email addresses
-punycode.toUnicode('xn--80ahgue5b@xn--p-8sbkgc5ag7bhce.xn--ba-lmcq');
+punycode.toUnicode('джумла@xn--p-8sbkgc5ag7bhce.xn--ba-lmcq');
 // → 'джумла@джpумлатест.bрфa'
 ```
 
@@ -117,7 +117,7 @@ punycode.toASCII('☃-⌘.com');
 
 // encode email addresses
 punycode.toASCII('джумла@джpумлатест.bрфa');
-// → 'xn--80ahgue5b@xn--p-8sbkgc5ag7bhce.xn--ba-lmcq'
+// → 'джумла@xn--p-8sbkgc5ag7bhce.xn--ba-lmcq'
 ```
 
 ### `punycode.ucs2`

punycode.js

@@ -95,12 +95,17 @@
 	 * function.
 	 */
 	function mapDomain(string, fn) {
+		var parts = string.split('@');
+		var result = '';
+		if (parts.length > 1) {
+			// In email addresses, only the domain name should be punycoded. Leave
+			// the local part (i.e. everything up to `@`) intact.
+			result = parts[0] + '@';
+			string = parts[1];
+		}
 		var labels = string.split(regexSeparators);
-		// Note: each label could still contain `@` in the case of an email address.
-		return map(labels, function(label) {
-			var parts = label.split('@');
-			return map(parts, fn).join('@');
-		}).join('.');
+		var encoded = map(labels, fn).join('.');
+		return result + encoded;
 	}
 
 	/**

tests/tests.js

@@ -236,7 +236,7 @@
 			{
 				'description': 'Email address',
 				'decoded': '\u0434\u0436\u0443\u043C\u043B\u0430@\u0434\u0436p\u0443\u043C\u043B\u0430\u0442\u0435\u0441\u0442.b\u0440\u0444a',
-				'encoded': 'xn--80ahgue5b@xn--p-8sbkgc5ag7bhce.xn--ba-lmcq'
+				'encoded': '\u0434\u0436\u0443\u043C\u043B\u0430@xn--p-8sbkgc5ag7bhce.xn--ba-lmcq'
 			}
 		],
 		'separators': [