setup.py - use extras_require for flask

Adding `moto` to the requirements file for my Django project pulls in `flask`, even though I don't need the server component. By making the `server` 'extra' then there is no hard requirement on flask.

setup.py

@@ -5,7 +5,6 @@ from setuptools import setup, find_packages
 install_requires = [
     "Jinja2",
     "boto>=2.36.0",
-    "flask",
     "httpretty==0.8.10",
     "requests",
     "xmltodict",
@@ -18,6 +17,8 @@ install_requires = [
 extras_require = {
     # No builtin OrderedDict before 2.7
     ':python_version=="2.6"': ['ordereddict'],
+
+    'server': ['flask'],
 }
 
 setup(