Consolidated fixes

Hi everyone,

Since there are many any pull requests pending on this project for the past 7-8 months, I've reviewed and consolidated most of them into a [branch on my fork](https://github.com/deepakprakash/django-activity-stream/tree/consolidated-fixes) for our own use.

Issues / Pull requests included:
* Django 1.5 Custom User Model support. (Pull requests #138, #133, #130)
* Stop bad data from breaking activity stream (#140)
* fix south migration warnings (#139)
* Load url tag from future (#137)
* Avoid deprecation warning (django 1.5) (#136)
* Added south migrations fix for issue when migrating a fresh project that uses Custom User Models. Done similar to [this](https://github.com/lukaszb/django-guardian/pull/116/files)

If you are looking for a version that is compatible with Django 1.5 and has other fixes, you can go ahead and use this branch. `pip install -e git+https://github.com/deepakprakash/django-activity-stream.git@consolidated-fixes#egg=django_activity_stream`

@justquick Guess this would be whenever you integrate into the original repository!

Cheers!