clean up the requirements a bit

requirements/core.txt

@@ -0,0 +1,18 @@
+Django==1.6.2
+MySQL-python==1.2.3
+South==0.8.4
+django-extensions==1.3.3
+httplib2==0.7.4
+logilab-astng==0.24.0
+logilab-common==0.58.1
+oauth2==1.5.211
+oauthlib==0.5.1
+ply==3.4
+pyflakes==0.5.0
+pylint==0.25.2
+python-dateutil==2.1
+requests==1.2.3
+requests-oauthlib==0.3.2
+six==1.1.0
+twython==3.0.0
+wsgiref==0.1.2

requirements/optional.txt

@@ -0,0 +1 @@
+psycopg2==2.5.2