Prevent aborting installation when settings is not available 

It's mostly fixes #165, Still I don't know why when `django-nonrel-1.5-beta` is installed, installation  fails, with this patch that error would be just logged.

The main problem with https://github.com/Alir3z4/mongodb-engine/pull/new/mongodb-engine-1.5-beta#L0L39 is that it caught `ImportError`, but other exceptions such as `ImproperlyCofigured`, `AttributeError` would abort installation.

Also I've changed `setuptools` to `distutils` since it's more standard to to the job, I have described the reason in it's commit message. But if you think it's not important to change I can revert it.

Also since I'm using this patch and [djangotoolbox][1] patch I have no critical issues.

[1]: https://github.com/django-nonrel/djangotoolbox/pull/40