RPC unlock Ethermint key and eth_sign

[austinabell]

• Implements Implement eth_sign #35 with key if enabled (eth_signTransaction is undocumented but may be useful to include signing a tx object)
• Attaches an unlocked key to the Public RPC API to be used for tx signing through rpc
• Sets up framework for personal_sign to sign data without an unlocked key on the server (is not enabled by default and would require Allow Configuration of RPC Modules #74 to enable)

To run:

make install 
rm -rf ~/.emint*
emintd init moniker --chain-id 3
emintcli config chain-id 3
emintcli config output json
emintcli config indent true
emintcli config trust-node true
emintcli emintkeys add austineth
testpass
testpass
emintcli rest-server --laddr "tcp://localhost:8545" --unlock-key austineth
testpass

curl -X POST --data '{"jsonrpc":"2.0","method":"eth_sign","params":["0x<USE ETHEREUM OUTPUT KEY FROM emintkeys add austineth>", "0xdeadbeaf"],"id":1}' -H "Content-Type: application/json" http://localhost:8545/rpc
## {"jsonrpc":"2.0","id":1,"result":"0xcd237050e74b9c2123312ef737715cc66e4f6cc30dd11497c98bafc486e5db56123b1b1d7eb7bbc5dc8c599e894170a138f922521131c0745326bbad8bd8d7e501"}