Added code for UserHandler disposal.

Currently, the bot doesn't remove unused `UserHandler`s. Each `UserHandler` holds user's backpack (can be couple hundred kilobytes).
I have added short method to remove `UserHandler` if it exists. This method is activated when user removes the bot. This means that in worst case scenario, the bot will hold the amount of `UserHandler`s equal to his friends list size.

Diff: https://github.com/scholtzm/SteamBot/commit/fc7b44d84a33fc5b9265f1b042d3eaa6bb5d56ab?w=1