Added a flag to allow storing promises

Classical use of cache:true in $http / $resources consists in storing the promise immediately.
By default angular-cache does not work this way and store the resolved value of the promise when it returns.

This is an annoying behavior change : 
 
 * imagine you've got a query taking 100ms to answer (slow backend/network)
 * your frontend calls the $resource twice in a 90ms timespan
 * you will do two requests instead of using the first promise to resolve the second call

This patch (against 2.x.x branch) adds a flag allowing to store the promise and let $resource do its job when the promise returns.