Applying license

[neumond]

Django-dia was derived from MIT-licensed code, but didn't specify explicit license for itself. I guess the best option would be to preserve existing MIT license. Therefore I'm asking all the contributors to approve this change, namely

@encolpe
@pauloxnet
@randomfish

Thank you!

[pauloxnet]

@neumond I see MIT license specified in https://github.com/neumond/django-dia/blob/master/setup.py and it's ok for me to preserve MIT license adding LICENSE file explicitly.