Merge branch 'melinath'

Merge branch 'melinath'

* melinath:
  Fixing style issues and an incorrect modification of .gitignore.
  Fixed the context variable leak in the blockcontainer tag.
  Fixed a bug where the ContainerNode had a nodelist_empty=None, whereas nodelist_empty is expected to be an iterable by line 314 of models.py. Also added 'nodelist_main' to the manage.py line 314 list of subnodelist possibilities and added a child_nodelists attribute to ContainerNode for possible forward-compatibility.
  First draft of blockcontainer functionality. Tries to reuse most of the code related to the original container tag by extending it, simply interpreting the values slightly differently.
  Added documentation and help text to models.py and containers.py to hopefully improve usability.