Missing various subscribe functions

[carmenlau]

In iOS we have various subscribe functions for different proposes, but in JS we only have global subscribe and subscribeTypingIndicator. We missed subscribeToMessagesInConversation and subscribeToConversation in JS

subscribe functions in iOS

-subscribeToUserChannelWithCompletion:
-subscribeToTypingIndicatorInConversation:handler:
-subscribeToMessagesInConversation:handler:
-subscribeToConversation: