More generic json loading allows to load a json from a specified folder rather than the "inventory" directory.
Also changed achievements location to <appid> root diectory
Added achievements support. Achievements are saved like real steam, see your inventory directory.(like items, uses a .json, tool can be used to dump achievements from steam public api).
You will need a public apikey. (See https://steamcommunity.com/dev)
Reworked item support to split inventory items to defined items.
Added check on directory in Local_Storage::file_exists.
Added check in get_filenames_recursive if last char is the path separator (some use cases were adding a path separator even if there were already one).
In base.h, added check on s and r, that will avoid an out-of-range vector exception on Visual Studio, same for network.cpp.
