Change install_requires to requires

'install_requires' is specific to setuptools which is not part of the
python standard library, the valid keywoard for distutils is 'requires'.
Check for netifaces availability

Since we are using 'distutils', it won't install dependency for us as we
could do with 'setuptools'.

It's not usefull to switch to 'setuptools' since, in order to install
'netifaces' it will try build an extension which requires to have
'python-dev' package, which is really not user-friendly.

Instead simply show an error message asking the user to install missing
dependency during setup.
Merge pull request #3 from maximeh/master

Various setup.py 'fixes'