When i install icalendar using pip directly from github, `unitest2` is not installed automatically. If i clone the source manually into a folder, obviously i have to install `unittest2` manually if i have an older version of Python.
```
vic@vic-desktop ~/projects/icalendar/src (master) » mkvirtualenv test
New python executable in test/bin/python
...
[test] vic@vic-desktop ~/projects/icalendar/src (master) » pip install git+git://github.com/collective/icalendar.git
Downloading/unpacking git+git://github.com/collective/icalendar.git
...
[test] vic@vic-desktop ~/projects/icalendar/src (master) » pip install nose
Downloading/unpacking nose
...
[test] vic@vic-desktop ~/projects/icalendar/src (master) » cd ~/projects/.venv/test/lib/python2.7/site-packages/icalendar
[test] vic@vic-desktop ~/projects/.venv/test/lib/python2.7/site-packages/icalendar » nosetests
E.EE.EE
======================================================================
ERROR: Failure: ImportError (No module named unittest2)
...
----------------------------------------------------------------------
Ran 7 tests in 0.039s

FAILED (errors=5)
```
Also, i think "explicit is better then implicit", better to not imply that unittest2 is installed, imo.