ethereum/go-ethereum#16734 introduced BlockHash to the FilterQuery
struct. However, ethclient was not updated to include BlockHash in the actual
RPC request.

The error produced when using a Parity RPC was the following:

ERROR: transaction did not get mined: failed to get tx for txid 0xbdeb094b3278019383c8da148ff1cb5b5dbd61bf8731bc2310ac1b8ed0235226: json: cannot unmarshal non-string into Go struct field txExtraInfo.blockHash of type common.Hash

DialContext allows users to pass a Context object for cancellation.
Close closes the underlying RPC connection.

* core/types: make Signer derive address instead of public key

There are two reasons to do this now: The upcoming ethclient signer
doesn't know the public key, just the address. EIP 208 will introduce a
new signer which derives the 'entry point' address for transactions with
zero signature. The entry point has no public key.

Other changes to the interface ease the path make to moving signature
crypto out of core/types later.

* ethclient, mobile: add TransactionSender

The new method can get the right signer without any crypto, and without
knowledge of the signature scheme that was used when the transaction was
included.

There is currently no simple way to obtain the network ID from a Client. 
This adds a NetworkID method that wraps the net_version JSON-RPC call.