unit test: improve CommonTestCase & GET tests & test database

CommonTestCase:
 - use urllib2 instead of urllib
 - add post method
 - add user agent as unittest
 - move setUp content to static data member

test_get_poll.py & test_get_polls.py
 - add 1 texture
 - remove setUp method

model/db.py
 - check user agent to use testunit database
Add unit test: post polls