Fix parsing of requirements.txt

During installation with setup.py no requirements are read from requirements.txt. The way `requirements.txt` is read right now, only one string containing all the package names is returned, resulting in `REQUIREMENTS` being `None`. 

Adding `.splitlines()` to `.read()` gives a proper list but also includes comments and other parameters to pip (which might be included), which can be seen below:

```
$ python setup.py --requires
['django-extensions', 'Django>=1.4.5', 'vobject', 'chardet', 'surlex', 'sorl-thumbnail', '', '# Test requirements', 'webtest', 'django-webtest']
```
A better alternative seems using pip's internals to parse the required packages:

```
$ python setup.py --required
['django-extensions', 'Django>=1.4.5', 'vobject', 'chardet', 'surlex', 'sorl-thumbnail', 'webtest', 'django-webtest']
```