Reorganize requirements.

Stab at issue #194

Have requirements.txt use setup.py, this way there is only one place of
requirements for users.

requirements-dev.txt

@@ -1,4 +1,6 @@
 -r requirements.txt
-flask
-boto
-httpretty
+mock
+nose
+sure<1.2.4
+coverage
+freezegun

requirements.txt

@@ -1,10 +1,2 @@
-coverage
-freezegun
-mock
-nose
-requests
-sure<1.2.4
-xmltodict
-dicttoxml
-werkzeug
-six
+#  Please add requirements to setup.py
+-e .

setup.py

@@ -3,13 +3,15 @@ from __future__ import unicode_literals
 from setuptools import setup, find_packages
 
 install_requires = [
+    "Jinja2",
     "boto",
     "dicttoxml",
     "flask",
     "httpretty>=0.6.1",
-    "Jinja2",
-    "xmltodict",
     "requests",
+    "xmltodict",
+    "six",
+    "werkzeug",
 ]
 
 import sys