Setting default value for kqList in Asset to null
Created cache for assets

cache module was created in assets application, and functions were defined in
order for it to be able to store if a course has bookable assets.
Models file was modified to invalidate the cache entry of a course each time a
kq is asigned or removed from an asset.
Allow an asset not to be selectable in any KnowledgeQuantum
Added file cache.py

That file should have been added in 443f3c6b5f95425955328fd7d59296cbcb329ba4
Created a new template tag to detect if a course has available assets

A template tag to detect if a course has available assets has been created.
File assets/utils.py was created with helper functions course_has_assets and
course_get_assets.

Base course template has been modified to include a link to the assets view.
Creating assets from admin interface
Creating reservations form admin interface
Add class AssetAvailability

New model class AssetAvailability added in order to simplify relation between
kwnowledgequantums and assets, and make it possible to store data such as when
an asset can be booked.

course_get_assets method was modified to handle the new schema.

Asset cache invalidation is yet to be adapted for the new schema.
Implemented unicode representation of AssetAvailability
Asset cache invalidation
Assets administration interface adapted to new schema
Asset resources adapted to new schema

Added AssetAvailabilityResource

Removed Asset filtering by kq, use AssetAvailability filtering instead
Initial assets overview page

Initial Assets overview page was created, once it's finished it should show the
current reservations made by the user, show the bookable assets, and in which
kqs and in what date they can be booked, and allow to book them.

This initial version does only show the assets which are avaliable in a course,
without allowing to book them or to see the current reservations.
Implemented unicode representation of reservation
Importing required modules in view.py
Displaying current reservations

Helper function user_course_get_reservation added