ClassCastException at parser.clj line 70.

Hi.

Feel free to ignore commit 9807ddc.  This is for some changes that I needed for myself, and I couldn't work out how to exclude them from the pull request.

About commit 6cb76ed, I was getting a ClassCastException when the type hint was included at the changed line : 
```
java.lang.ClassCastException: selmer.node.TextNode cannot be cast to selmer.node.INode
 at selmer.parser$render_template.invoke (parser.clj:70)
    selmer.parser$render_file.doInvoke (parser.clj:99)
```

I only ever say the exception at the REPL, never in execution from the command line, which was weird.

Greg.