Improve generate genesis script

[NicolasMahe]

• This PR merge the previous 3 gen scripts to one that do the same and more
• The new script writes to the disk in engine/.genesis folder all necessary info (includes private keys) for not only 1 validator, but other validator as well as simple client to be started easily with right config
• The script has a new flag --gen-genesis that generate a new genesis before starting the engine. It should be passed only once if you want to connect multiple engine to each other. Set the env MESG_VALIDATOR_NUMBER to 2 or more if you want to create a network with multiple validator.
• Use the flag --genesis_account to copy the all accounts private info to the engine. Only useful to execute transaction with a validator account. (will be deleted when it's possible to import an account from private key or mnemonic).
• Of course, the flag --validator is still present and copy one validator private info to the engine. You will also need to provide the right validator name using the env MESG_NAME (the name are outputted when using flag --gen-genesis. Make sure also to set another port with MESG_TENDERMINT_VALIDATOR_PORT if you want to start multiple validator on the same computer.

Example of 2 validators and 2 clients:

• validator Event store #1 bob

MESG_VALIDATOR_NUMBER=2 ./dev --validator --gen-genesis

• validator Events consensus #2 alice

MESG_SERVER_PORT=50053 MESG_PATH=~/.mesg2 MESG_NAME=alice MESG_TENDERMINT_VALIDATOR_PORT=26666 ./dev --validator 

• client Event store #1 charles

MESG_SERVER_PORT=50054 MESG_NAME=charles MESG_PATH=~/.mesg3 ./dev

• client Events consensus #2 david

MESG_SERVER_PORT=50055 MESG_NAME=david MESG_PATH=~/.mesg4 ./dev

closes #1385