A generic resource caching mechanism

Most (distant) resources don't change that often, and doing a request each time can be costly (especially if this request goes through the network on a slow connection).
Moovida could implement a generic and transparent resource caching mechanism, for instance at the resource manager level, for the get method.
This mechanism should be flexible enough, configurable, easy to override in resource providers, and even easy to completely de-activate for debugging purposes.

Such a mechanism will include:
 - an id system: could be the URIs of the resources
 - an invalidation strategy (probably time-based with a good default value, and easy to override locally)