added support for + operator for calc functions

Author: Merrifield, Jay

src/css/Parser.js

@@ -712,14 +712,14 @@ Parser.prototype = function(){
 
                 /*
                  * operator
-                 *  : '/' S* | ',' S* | '*' S* | '-' S* /( empty )/
+                 *  : '/' S* | ',' S* | '+' S* | '*' S* | '-' S* /( empty )/
                  *  ;
                  */    
 
                 var tokenStream = this._tokenStream,
                     token       = null;
 
-                if (tokenStream.match([Tokens.SLASH, Tokens.COMMA, Tokens.STAR, Tokens.MINUS])){
+                if (tokenStream.match([Tokens.SLASH, Tokens.COMMA, Tokens.PLUS, Tokens.STAR, Tokens.MINUS])){
                     token =  tokenStream.token();
                     this._readWhitespace();
                 } 

tests/css/Validation.js

@@ -493,7 +493,7 @@
             "1px",
             "1%",
             "calc(100% - 5px)",
-            "calc(100% - 1em)",
+            "calc(100% + 1em)",
             "calc(100%/6)",
             "calc(10%*6)",
             "inherit"