* (rpc) [\#231](https://github.com/ChainSafe/ethermint/issues/231) Implement NewBlockFilter in rpc/filters.go which instantiates a polling block filter
* Polls for new blocks via BlockNumber rpc call; if block number changes, it requests the new block via GetBlockByNumber rpc call and adds it to its internal list of blocks
* Update uninstallFilter and getFilterChanges accordingly
* uninstallFilter stops the polling goroutine
* getFilterChanges returns the filter's internal list of block hashes and resets it
* for a given filter, look through each block for transactions. If there are transactions in the block, get the logs from it, and filter using the filterLogs method
* eth_getLogs and eth_getFilterChanges for log filters use the same underlying method as eth_getFilterLogs
* update HandleMsgEthereumTx to store logs using the ethereum hash