Add override details to validate's docstring
copy over b63bb0 import fixes to xblock/reference/plugins.py
version 1 of the user service, supporting get_current_user
Merge pull request #273 from edx/jbau/user-service

version 1 of the user service, supporting get_current_user
Merge pull request #270 from edx/andya/improve-validate-docstring

Add override details to validate's docstring
XBlock and Asides common behaviors
Asides special behaviors
Pin astroid to fix tests
Merge pull request #269 from edx/dhm/view_decorator

XBlock and Asides common behaviors
Support Python 3.3 and 3.4

Uses `six` for compatibility