Add 180 RINGING request in onCallReceived method

[glocore]

fixes #61
Wasn't able to hear a ringing tone when calling TO react-native-pjsip. Turns out that a 180 RINGING request was not being sent, and so the caller could not hear the ringing.

[datso]

This action should be optional, it would be better to have additional params in answerCall method where you can pass 180 code for ringing.

[csranasinghe]

I found a way to do the same in android. But have a question about state changes. In IOS also after we send the 180 answer, the state changes to PJSIP_INV_STATE_EARLY not to PJSIP_INV_STATE_INCOMING is it ok? In android actually I didnt get to PJSIP_INV_STATE_INCOMING state in any moment. But after sending 180 then I get the state change as PJSIP_INV_STATE_EARLY. Is this behavior normal? If that's normal then I can commit the android change.