Fix encoding issues in PDFTemplateResponse with python 2.x

Issue #13 fixed the problem for python 3.x but not for python 2.x. I fixed the problem and also updated the tests to test PDFTemplateResponse and PDFTemplateView (the old tests passed but in fact, they were not testing unicode content at all).

This also solves issue #51. Let me now if I missed something.
In fact, the problem is with django 1.4 (that doesn't have smart_text, only smart_unicode). Do you suggest me to just comment in the import that smart_text isn't available in django 1.4 or, in the PDFTemplateResponse, I must rely in a code like the above?

```python
        try:
            from django.utils.encoding import smart_text
            content = smart_text(template.render(context))
        except ImportError:
            # Django 1.4 and python 2.x compatible code
            from django.utils.encoding import smart_unicode
            content = smart_unicode(template.render(context))
```