compatibility with python 2

Add support of the threaded worker on python 2.7. python 2.7 has no
futures module. With this change the compatibility module is installed.

setup.py

@@ -10,6 +10,7 @@ import sys
 
 from gunicorn import __version__
 
+
 CLASSIFIERS = [
     'Development Status :: 4 - Beta',
     'Environment :: Other Environment',
@@ -61,6 +62,11 @@ class PyTest(Command):
         raise SystemExit(errno)
 
 
+REQUIREMENTS = []
+if sys.version_info[0] == 2:
+    REQUIREMENTS.append('futures')
+
+
 setup(
     name = 'gunicorn',
     version = __version__,
@@ -80,6 +86,8 @@ setup(
     tests_require = tests_require,
     cmdclass = {'test': PyTest},
 
+    install_requires = REQUIREMENTS,
+
     entry_points="""
 
     [console_scripts]