Fix loading of esprima.js.

Let Esprima have its window object - otherwise it'll just crash.

lib/jsduck/esprima.rb

@@ -14,7 +14,13 @@ module JsDuck
     def initialize
       @v8 = V8::Context.new
       esprima = File.dirname(File.dirname(File.dirname(File.expand_path(__FILE__))))+"/esprima/esprima.js";
+
+      # Esprima attempts to assign to window.esprima, but our v8
+      # engine has no global window variable defined.  So define our
+      # own and then grab esprima out from it again.
+      @v8.eval("var window = {};")
       @v8.load(esprima)
+      @v8.eval("var esprima = window.esprima;")
     end
 
     # Parses JavaScript source code using Esprima.js