Merge pull request #369 from campusparty/stable/0.2.x

Add manager command to export containerboxes data
Adds main_image foreign key to channel
Merge pull request #370 from mauler/feature-py26

Fixes for python 2.6 compatibility
Merge pull request #371 from mauler/feature-channel-main_image

Adds main_image foreign key to channel
run test in python 2.6 and 2.7
Fix Channel migration
Removes python 2.6 travis tests
Makes compatible with python 2.6