Feature/base template

Potential fix for #29.  It basically keeps the existing templates, which inherit a very simple base template.  Only changes there are that 1) the templates will render as valid html, and 2) i added header titles.

i also made a branch with a sample project which demonstrates how the base template might be overridden to use admin templates: https://github.com/smcoll/django-simple-import/tree/feature/sample-project

If you like that, i can create another pull request.