Remove usage of _.keys.

relates to #68

src/lang/lang_extensions.ts

@@ -37,7 +37,7 @@ namespace chevrotain.lang {
         private _state = {}
 
         keys():string[] {
-            return _.keys(this._state)
+            return utils.keys(this._state)
         }
 
         values():V[] {

src/parse/grammar/lookahead.ts

@@ -121,7 +121,7 @@ namespace chevrotain.lookahead {
                 })
             })
 
-            let altsIndicesTokenAppearsIn = _.map(_.keys(altsCurrTokenAppearsIn), (index) => {
+            let altsIndicesTokenAppearsIn = _.map(utils.keys(altsCurrTokenAppearsIn), (index) => {
                 return parseInt(index, 10) + 1
             })
 

src/utils/utils.ts

@@ -13,4 +13,8 @@ namespace utils {
     export function isEmpty(arr:any[]) {
         return arr.length === 0
     }
+
+    export function keys(obj:any) {
+        return Object.keys(obj)
+    }
 }