eth_getTransactionCount implementation

[austinabell]

• Implements Implement eth_getTransactionCount #29

Currently runs into same issue as previous methods, indicated with #89 so this can be merged in and all of the issues can be handled at once or this can be left and the fix can be included in this PR

To test:

1. Run the Ethermint node how it is normally run, I ran:

make install 
rm -rf ~/.emint*
emintd init moniker --chain-id testchain
emintcli config chain-id testchain
emintcli config output json
emintcli config indent true
emintcli config trust-node true
emintcli keys add austin
<password>
<password>
emintd add-genesis-account $(emintcli keys show austin -a) 1000photon,100000000stake
emintd gentx --name austin
<password>
emintd collect-gentxs
emintd validate-genesis
emintd start --pruning=nothing

2. Start rest server

emintcli rest-server --laddr "tcp://localhost:8545"

3. Send request:

curl -X POST --data '{"jsonrpc":"2.0","method":"eth_getTransactionCount","params":["0xc94770007dda54cF92009BFF0dE90c06F603a09f","0x2"],"id":1}' -H "Content-Type: application/json" http://localhost:8545/rpc

Historical queries still not available even with pruning flag and errors on current block