Support for answer posts

Hello, 

I know this library is quite old and it also looks quite obsolete (especially because of the fact that there is the Tumblr API v2).
But in case anybody still wants to use it (like I did for a project) it's currently crashing when there are any answer posts in the blog. This should fix the issue. Tests aren't included though, I hope it's ok.

Thanks for the library,
Evotopid