Add a getDeclaredMethod() function

[Sam Lu]

Would you add the following API?
Method getDeclaredMethod(String name, Class...<?> parameterTypes)

The reasons are:

1. we can cache the founded Method in our own class so it does not have to look up methods for each invoke() call
2. easier for developers to find the polymorphism function

Public the checkArgsForInvokeMethod() function is also an acceptable workaround.