core, eth: added json tag field for proper unmarshalling
core, eth: added json tag field for proper unmarshalling

According to our own instructions the genesis config attribute should be
"config". The genesis definition in the go code, however, has a field
called `ChainConfig`. This field now has a `json:"config"` struct tag so
that the json is properly unmarshalled.

This fixes #2482

maran/go-ethereum,maran/go-ethereum,maran/go-ethereum,maran/go-ethereum,maran/go-ethereum,maran/go-ethereum,maran/go-ethereum,maran/go-ethereum,maran/go-ethereum