SteamBot Rewrite

Here's the SteamBot rewrite I've been working on. Any further changes are pending your feedback and testing.

The following is a list of what was changed (done to the best of my ability):
* You can set which inventories to be fetched in `OnLoginCompleted()` when a new `GenericInventory` instance is created.
* Removed `AdminUserHandler.cs` and `SimpleUserHandler.cs` since those were more TF2-oriented. If someone would like to create a generic version of AdminUserHandler and send it as a pull-request, that would be great.
    * Renamed `SteamTradeDemoHandler.cs` to `SimpleUserHandler.cs`
* In the Trade session, the `amount` variable was hard-set to 1, but some games (such as Spiral Knights) require it. `amount` was also added to the POST data sent in a trade, but this is untested.
* Friends-only inventories are no longer a problem when outside trades. A cookie is sent when fetching generic inventories. Unfortunately this doesn't work for the `GetPlayerItems` api.
* `GenericInventory.Inventory.Item` is passed into `OnTradeAddItem` (and etc) instead of `Schema.Item` and `Inventory.Item`.
* Created folders for Inventories and Schemas
    * Renamed `Inventory.cs` and `Schema.cs` to `TF2Inventory.cs` and `TF2Schema.cs`. Fetching schemas is now done on demand in `OnLoginCompleted`.
    * Also added `Dota2Inventory.cs` and `Dota2Schema.cs`
* Trades now initialize instantly, no longer waiting for all inventories to be fetched first. This avoids the user seeing "Bot is loading..." all the time.
* `Trade.MyInventory` and `Trade.OtherInventory` is now public.
* Trade events are now queued. This prevents exceptions from being thrown such as sending a trade message after the trade has expired, along with ensuring that events are handled in order.
* Removed `ForeignInventory.cs`. Handling private inventories is done automatically in the trade session.

For a full list of changes, you should look through [the commits](https://github.com/waylaidwanderer/SteamBot/commits/GenericRewrite).