add: implement concat

Author: qiao

src/immutable_sequence.js

@@ -152,6 +152,15 @@
     return new ImmutableSequence(this.tree_.removeLast());
   };
 
+  /**
+   * Concatenate the sequence with another one.
+   * @param {ImmutableSequence} other
+   * @return {ImmutableSequence}
+   */
+  ImmutableSequence.prototype.concat = function (other) {
+    return new ImmutableSequence(this.tree_.concat(other.tree_));
+  };
+
   /**
    * Split into two sequences at the given index.
    * @param {number} i