Add dns extras in setup.py

Add an `extras_require` section in ` setup.py` for installing `pyDNS` dependency  if not on Windows, and installed like this:

    pip install validate-email[dns]

Author: ankostis

setup.py

@@ -10,5 +10,8 @@
       long_description=open('README.rst').read(),
       keywords = 'email validation verification mx verify',
       url = 'http://github.com/syrusakbary/validate_email',
+      extras_require = {
+            'dns"sys_platform != "win32"': ['pyDNS']
+      },
       license = 'LGPL',
     )