Introduce functions in OpenlmisCachedResource that fetch data both online and offline

In OpenlmisCachedResource we should have functions that allow us to fetch the data no matter if we are online or offline. The functions should discover if we are online or offline.

If we are online, it fetches the data from the remote resource (server) with If-Modified-Since param. If there are changes it updates them in the cache. Then we return the resource from the cache.
If we are offline, it attempts to fetch the data from the cache.