Add penTrails library

libraries/penTrails.xml

@@ -0,0 +1 @@
+<blocks app="Snap! 4.0, http://snap.berkeley.edu" version="1"><block-definition s="pen trails" type="reporter" category="pen"><header></header><code></code><inputs></inputs><script><block s="doReport"><block s="evaluate"><block s="reportJSFunction"><list></list><l>return new Costume(&#xD;  this.parentThatIsA(StageMorph).trailsCanvas&#xD;);</l></block><list></list></block></block></script></block-definition><block-definition s="set pen trails to: %&apos;costume&apos;" type="command" category="pen"><header></header><code></code><inputs><input type="%s" readonly="true"></input></inputs><script><block s="doRun"><block s="reportJSFunction"><list><l>cst</l></list><l>var stage = this.parentThatIsA(StageMorph);&#xD;stage.trailsCanvas = cst.contents;&#xD;stage.changed();</l></block><list><block var="costume"/></list></block></script></block-definition></blocks>