Improve caching system

[julien4215]

• We are currently using SharedPreferences to cache requests. This is a bad idea as SharedPreferences is not conceived to store large amount of data and can lead to Out Of Memory errors (Remove caching #397)
• Currently nothing get deleted in the cache, we should clear the cache after some period of time
• We are not caching images and pdf. We don't have to but we should at least take care of the case were the user has no connectivity.