add constructor to accept custom http client. (#591)
add constructor to accept custom http client. (#591)

Newly added constructor accepts customized http client which replaces
default http client used.

```
public MinioClient(String endpoint, int port, String accessKey,
                   String secretKey, String region, boolean secure,
                   OkHttpClient httpClient)
```

Fixes #589
balamurugana/minio-java,minio/minio-java,vadmeste/minio-java