Add basic IE8 support

Currently I've only made enough fixes so a few basic smoke tests work, e.g.:

```javascript
var tmpl = new nunjucks.Template('Hello {{ username }}');
var rendered = tmpl.render({username: 'James'});
expect(rendered).to.be("Hello James");
```

Also note that in this pull request, I have implemented a `lib.inArray()` helper in 77d6f5e4e700e2b21f7873bad2f4de7a52763c1a, since `Array.prototype.indexOf()` doesn't exist on IE8. An alternative to adding our own ES5-fallback helper functions is to simply require that IE8 clients include [es5-shim](https://github.com/kriskowal/es5-shim).