Merge PR: add ethclient (#139)

* add evmclient.ethclient

* add ethclient.go

* keep same with master

* add rpc interface

* fix callContext

* fix code rule

* rename main() in sample

* del unusable code

* wrapped rpcClient

* fix sample

* move samples

ethclient.go

@@ -0,0 +1,23 @@
+package gosdk
+
+import (
+	"context"
+	"github.com/ethereum/go-ethereum/ethclient"
+	"github.com/ethereum/go-ethereum/rpc"
+)
+
+type rpcClient struct {
+	*rpc.Client
+}
+
+type ethClient struct {
+	*ethclient.Client
+	*rpcClient
+}
+func NewEthClient(ctx context.Context, rawurl string) (*ethClient, error) {
+	c, err := rpc.DialContext(ctx, rawurl)
+	if err != nil {
+		return nil, err
+	}
+	return &ethClient{ethclient.NewClient(c), &rpcClient{c}}, nil
+}