added getHelper for use in aliasing

This would help in aliasing/overriding helpers from external libraries.
```
var old = Handlebars.getHelper('example');
Handlebars.registerHelper("example-old", old);
//...
var newFunc = function(){};
Handlebars.registerHelper("example", newFunc);
```
or
```
var old = Handlebars.getHelper('example');
Handlebars.registerHelper("example", function(value) {
  //do something additional
  return old(value);
});
```