Add a pipchecker command for reporting on out-of-date requirements.

This handles the common cases of pypi and github, but currently not a private pypi or use --extra-index-url. I use this in a monthly cronjob to report on out-of-date requirements by email.

For more than 50 github api calls per hour, pipchecker supports authentication with the github api.

Pipchecker depends on the requests library and complains loudly if it isn't installed.

Here is sample output:

```
$ python manage.py pipchecker

mock 1.0b1                               1.0.1 available
Fabric 1.4.3                             1.5.3 available
django-compressor 1.0.1                  release is not on pypi (check capitalization and/or --extra-index-url)
django-podcasting 0.9.4                  repo is not frozen
Django 1.6.dev20121210223451             393c268e72 is not the head of any branch.
django-faq 0.1.0                         5008320355 is not the head of any branch.

Only pypi and github based requirements are supported.
django-pagination 1.0.7                  is not a pypi or github requirement
django-ajax-validation 0.1.4             is not a pypi or github requirement
```

with this example requirements.txt:
```
mock==1.0b1
BeautifulSoup==3.2.1
Fabric==1.4.3
ipython==0.13.1
django-compressor==1.0.1
git+https://github.com/rizumu/django-podcasting.git#egg=django-podcasting
git+git://github.com/django/django.git@393c268e725f5b229ecb554f3fac02cfc250d2df#egg=Django
git+git://github.com/howiworkdaily/django-faq@50083203559a788d36c25fc4fe65a984c1d739e6#egg=django-faq
-e git+ssh://git@gitlab.internet.int/rizumu/django-pagination.git@rizumu-1.0.7#egg=django_pagination
-e git+ssh://git@gitlab.internet.int/rizumu/django-ajax-validation.git@rizumu-7bb26#egg=django_ajax_validation
```