Don't use "this" inside peg.generate

This change makes it possible to use "generate" as a module function,
e.g. like this:

  var generate = require("pegjs").generate;
  var parser = generate(...);

Fixes #433.

lib/peg.js

@@ -41,13 +41,13 @@ var peg = {
 
     var plugins = "plugins" in options ? options.plugins : [],
         config  = {
-          parser: this.parser,
-          passes: convertPasses(this.compiler.passes)
+          parser: peg.parser,
+          passes: convertPasses(peg.compiler.passes)
         };
 
     arrays.each(plugins, function(p) { p.use(config, options); });
 
-    return this.compiler.compile(
+    return peg.compiler.compile(
       config.parser.parse(grammar),
       config.passes,
       options