Use unittest2 for Python 2.6 compatibility

test_requirements.txt

 httreplay==0.1.4
 coveralls==0.2
+unittest2==0.5.1

tests/config.py

 import os
-from unittest import TestCase
+import sys
+from unittest2 import TestCase
 
 from httreplay import start_replay, stop_replay
 from httreplay.utils import filter_headers_key

tests/test_api.py

-import unittest
+import unittest2 as unittest
 import random
 from time import sleep
 import os

tests/test_auth.py

-import unittest
+import unittest2 as unittest
 
 from config import *
 from tweepy import API, OAuthHandler

tests/test_cursors.py

-import unittest
+import unittest2 as unittest
 
 from tweepy import API, Cursor
 

tests/test_resultset.py

-import unittest
+import unittest2 as unittest
 
 from tweepy.models import ResultSet
 

tests/test_streaming.py

 from time import sleep
-import unittest
+import unittest2 as unittest
 
 from tweepy.api import API
 from tweepy.models import Status

tests/test_utils.py

-from unittest import TestCase
+from unittest2 import TestCase
 
 from tweepy.utils import *