High-level bindings should have more implicit derived-to-base conversions

[jfirebaugh]

#26 was a good start, but we should go further:

• Object<T>::Set(env, field, value) should allow an implicit derived-to-base conversion from value's type (derived) to the type of the field (base).
• Object<T>::Call(env, method, arg) should allow an implicit derived-to-base argument from arg's type (derived) to the corresponding argument type of the method (base).
• ...and so on

Right now, template parameters constrain these types to be identical. For example, if you try Call a method that's expecting a Object<> parameter, and pass a jni::String, you get an error "no matching member function for call to 'Call'", "candidate template ignored: deduced conflicting types for parameter 'Args' (<jni::Object<jni::ObjectTag>> vs. <jni::Object<jni::StringTag>>)".