Adding hashability to SteamApp, SteamAchievement to allow for easier comparison

it will allow for sets to be made using the steamapp objects, and for comparisons to be done on those sets, for example

```python
import steamapi
steamapi.core.APIConnection(api_key=APIKEY)
SteamUser = steamapi.user.SteamUser

for x in set(SteamUser(userurl="abc")).intersection(SteamUser(userurl="dfg")):
    print(x)
```

to find out what games are similar between two people