Make db_manager.DaoSession private

this is a first step toward a Session object for all dao functions

a mocked_dao_session decorator has been added to the db_manager module to
decorate test functions that need a mocked session instance.

The interesting stuff in this commit is in xivo_dao.helpers.db_manager other
changes are mostly replacing DaoSession patching with @mocked_dao_session
move mocked_dao_session to its own test module
add missing file in previous commit
Merge branch 'master' into private_daosession

Conflicts:
	xivo_dao/data_handler/func_key/tests/test_dao.py
Merge pull request #14 from xivo-pbx/private_daosession

Make db_manager.DaoSession private