feat(@embark/accounts-manager): Get alternative coinbase address
feat(@embark/accounts-manager): Get alternative coinbase address

In dev mode, accounts are funded per the blockchain accounts config. In specific situations, there may not be enough funds on the account returned by `eth_coinbase`. In that case, and in the case when `eth_coinbase` returns `0x0` (or equivalent), loop through all accounts and find the one that has the most funds and use that as the coinbase account.

iurimatias/embark-framework,iurimatias/embark-framework