Added getSteamLevel method (squashed)

- account IDs -> SteamIDs
- Added missing semicolon
- Fixed indentation
- SteamID not Steam ID
- Updated readme to reflect that callback is required for getSteamLevel
- Updated readme
- Accept SteamIDs instead of account IDs
- Updated readme
- Removed redundant steamLevels event
- Updated readme
- Added getSteamLevel method (with callback) and steamLevels event