Add the 'closed' argument to the lines method.

* When 'closed' is true, an 'h' operator is included in the PDF code generated
  by the lines method, so the subpath is closed.
* The option is used in the triangle method, so it generates a closed triangle,
  with all the corners rendered with the same join style.
* @param descriptions for 'style' and 'closed' were added to the documentation
  of the lines method.

jspdf.js

@@ -1247,12 +1247,14 @@ PubSub implementation
         @param {Number} x Coordinate (in units declared at inception of PDF document) against left edge of the page
         @param {Number} y Coordinate (in units declared at inception of PDF document) against upper edge of the page
         @param {Number} scale (Defaults to [1.0,1.0]) x,y Scaling factor for all vectors. Elements can be any floating number Sub-one makes drawing smaller. Over-one grows the drawing. Negative flips the direction.
+        @param {String} style One of 'S' (the default), 'F', 'FD' or 'DF'.  'S' draws just the curve. 'F' fills the region defined by the curves. 'DF' or 'FD' draws the curves and fills the region. 
+        @param {Boolean} closed If true, the path is closed with a straight line from the end of the last curve to the starting point. 
         @function
         @returns {jsPDF}
         @methodOf jsPDF#
         @name lines
          */
-        API.lines = function (lines, x, y, scale, style) {
+        API.lines = function (lines, x, y, scale, style, closed) {
             var undef, _first, _second, _third, scalex, scaley, i, l, leg, x2, y2, x3, y3, x4, y4;
 
             // Pre-August-2012 the order of arguments was function(x, y, lines, scale, style)
@@ -1310,6 +1312,11 @@ PubSub implementation
                     );
                 }
             }
+
+            if (closed == true) {
+                out(' h');
+            }
+
             // stroking / filling / both the path
             out(style);
             return this;
@@ -1366,7 +1373,8 @@ PubSub implementation
                 x1,
                 y1, // start of path
                 [1, 1],
-                style
+                style,
+                true
             );
             return this;
         };