Custom archive organizations + configurable default archive format(s)

Checkpoint of code to handle custom archive organizations

Checkpoint ... organization support mostly working apart from unit tests

Completed the unit test changes, and added some testing for 'tar' format

Added documentation and implemented 'filtering out'.  Also fixed a bug
in the way I was wrapping the mapper functions.

First cut at wiring up the new download stuff to to UI

Conflicts:
	tardis/tardis_portal/templates/tardis_portal/ajax/experiment_description.html

Resolved:
	merged button styling change from master

Documentation tweak

Updated UI for download.  Better "download experiment" button & download selected datasets / datafiles now download in "default" org and archive type

Bug fix download in a dataset

Conflicts:

	tardis/tardis_portal/views.py

get_download_url shouldn't croak if there is currently no accessible replica

Use better names for some config and context parameters

Unit tests for download format/organization changes

The Python 2.6 & 2.6 tarfile modules handle empty TAR files differently

Compilation error

If the set of files that would ultimately be included in the archive
is empty, give an error ... not an empty archive.

Revert "Added back some dependencies that are needed for python 2.6 (at least)"

This reverts commit 6bf144ad5ec5285b355327deab67760684582101.
Change the download error message / code for selective download too.
Merge pull request #249 from crawley/archive-organizations

Custom archive organizations + configurable default archive format(s)