feature: detect subscription support in provider

[simon-jentzsch]

currently in a lot of lines in the code the provider is checked whether is supports subscribe. This is done by simply checking, if the on-function exists

if (provider.on)
   ...
else
   ...

Since we also use different Wrappers, which even dynamically change the inner provider, this means, we must add and delete a function during runtime depending on the inner provider.
I would rather suggest a function supportsSubscription(). This would make it cleaner and easier to implement and even delegate this call to the inner wrapper.

if (provider.supportsSubscription())
   ...
else
   ...