Ethereum API filter methods

[aakoshh]

Part of #342

Implement the following methods:

• eth_newBlockFilter
• eth_newFilter
• eth_newPendingTransactionFilter
• eth_uninstallFilter
• eth_getFilterChanges
• eth_getFilterLogs

These methods require a subscription by Tendermint using the WebSocketClient, which supports all the regular Client methods as well as the SubscriptionClient ones, so it can replace the HttpClient, but it needs a different URL to connect to.

The eth_ methods themselves don't require WebSocket connections, they are polled based on a filter ID. The ones based on subscriptions will be implemented separately.