Fix indentation in Blender exporter script

This pull request fixes the indentation in the Blender exporter script.

Blender exporter script in the dev branch uses a mix of tabs in certain places, while the rest of the file uses spaces. It causes an error.

![screen shot 2014-04-07 at 1 16 04 pm](https://cloud.githubusercontent.com/assets/84005/2628779/450fdd0c-be2d-11e3-8c07-50504206b93d.png)