Adds psycopg2 as an extra

can install psycopg2 by using the command line syntax

```pip install -e .[sql]```

Author: kdschlosser

setup.py

@@ -59,10 +59,13 @@ def run(self):
     install_requires=[
         'requests'
     ],
+    extras_require = {
+        'sql':  ["psycopg2"]
+    }
     classifiers=[
         'Intended Audience :: Developers',
     ],
     cmdclass={
         'publish': PublishCommand,
     }
-)
+)