Connect to eth node

[smoelius]

This PR enables Manticore to connect to an Ethereum node and generate symbolic transactions for a deployed contract using the contract's on-chain storage. Manticore obtains the contract's storage values using the node's JSON RPC interface.

This PR specifically adds two command line options:

• --rpc URL: specify the URL of an Ethereum node to connect to. Must be of the form IP:PORT
• --txtarget TXTARGET: specify the address of a deployed contract to target

This PR further adds an optional world_state parameter to ManticoreEVM's constructor. One can connect to an Ethereum node from within a Manticore script as follows:

from manticore.ethereum import ManticoreEVM
from manticore.platforms.evm_world_state import RemoteWorldState

m = ManticoreEVM(world_state=RemoteWorldState(URL))