Testing reworked: testing against multiple environment settings and travis integration

Django-userena still has some problems when used with Django 1.6.x. I wanted to contribute on this issue but I found that testing easily against different versions of django was impossible. So I reworked testing for userena.

Changes made:
- Testing app configuration is moved to separate `runtests` submodule (demo application and testing application are now separate because they always should be so). This is pattern successfuly used in other projects ([django-rest-framework](https://github.com/tomchristie/django-rest-framework/tree/master), [django-oauth-plus](https://bitbucket.org/david/django-oauth-plus/src)) and solves few issues while tox-testing with new django (>=1.6.0) test runner
- Added tox configuration. Django userena can now be tested against different vesions of django and python with just one line in console (tox)
- Added .travis.yml configuration file so django-userena can now benefit from Travis CI
- Fixed few minor issues related to testing on some versions of django like missing `404.html` template in application for django 1.4.x or changed email tests to use built-in templates insted of those from demo application
- Refactored tests submodules in order to be discoverable by django 1.3.x and >=1.6.x test runners (`test_module`). This pattern should be always followed from now.
- removed ProfileTestCase because its obsilete since `runtests.settings` has `userena.tests.profiles` in in `INSTALLED_APPS` with some profile defined.
- Fixed issue on messing global settings object during `test_utils` tests. That issue made many followng tests to fail without reason on Django 1.6.1

Notes:
- ~~tests shows that a bunch of tests fails in django 1.6.1~~. All test passes on Django 1.4.10, 1.5.5, 1.6.1
- tox has `usedevelop = True` set because without that it won't be able to run tests in django 1.6.x