feat(setup): Add dns extras for pyDNS

+ Add an `extras_require` section in ` setup.py` for installing `pyDNS`
dependency, for non Windows.
+ Updated docs.

Author: ankostis

README.rst

@@ -20,6 +20,10 @@ For check the domain mx and verify email exits you must have the `pyDNS` package
 
     pip install pyDNS
 
+or install them both in one step from the beginning::
+
+    pip install validate-email[dns]
+
 
 USAGE
 =====
@@ -50,4 +54,4 @@ Check if the host has SMTP Server and the email really exists::
 
 TODOs and BUGS
 ==============
-See: http://github.com/syrusakbary/validate_email/issues
+See: http://github.com/syrusakbary/validate_email/issues

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',
     )