implement filters in the Client

[simon-jentzsch]

This should create a id inside the Client and triggers a request
whenever eth_getFilterChanges is called.

Here we should support

• eth_newFilter
  -> call eth_getLogs with the last checked blockNumber

• eth_newBlockFilter
  -> call eth_getBlockNumber and the fetch all blocks after the last checked blockNumber

• eth_newPendingTransactionFilter ( not supported )

• tests implemented