Validate input of RPC requests

[pkakelas]

Depends #13

example request:

{
	"jsonrpc":"2.0",
	"method":"eth_getBalance",
	"params":[
		"0x407d73d8a49eeb85d32cf465507dd71d507100c1", 
		"a random block opt"
	],
	"id":1
}

response:

{
    "jsonrpc": "2.0",
    "id": 1,
    "error": {
        "code": -32602,
        "message": "invalid argument 1: block option must be a valid 0x-prefixed block hash or hex integer, or \"latest\", \"earliest\" or \"pending\""
    }
}