Systematically replace Base64 signing function

[Desrever Nu]

/Users/advanced/Desktop/nubot/src/main/java/com/nubits/nubot/trading/wrappers/AltsTradeWrapper.java:30: warning: Base64 is internal proprietary API and may be removed in a future release
import com.sun.org.apache.xerces.internal.impl.dv.util.Base64;
                                                      ^
/Users/advanced/Desktop/nubot/src/main/java/com/nubits/nubot/trading/wrappers/AltsTradeWrapper.java:655: warning: Base64 is internal proprietary API and may be removed in a future release
                key = new SecretKeySpec(Base64.decode(secret), SIGN_HASH_FUNCTION);
                                        ^
/Users/advanced/Desktop/nubot/src/main/java/com/nubits/nubot/trading/wrappers/AltsTradeWrapper.java:660: warning: Base64 is internal proprietary API and may be removed in a future release
                sign = Base64.encode(mac.doFinal(hash_data.getBytes(ENCODING)));

see http://stackoverflow.com/questions/21904682/how-to-avoid-warning-for-the-base-64