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.
What I try was to use smart_text if it is available or smart_unicode when not.

It won't be availble in django 1.4, but django 1.5 was the first python 3.x compatible version. So, if there is smart_text, it can be used for both py2 and py3 but, if not, django don't even support py3, understand?

I must admit to rename smart_unicode import as smart_text became not so legible. I will change the approach.