c-generic/libpubnub: Proper curl error handling all over the map

This includes some slight semantics tweaks, also in pubnub-sync, and enhanced error reporting in the example demo.
c-generic/libpubnub: Rename callbacks to a consistent scheme

pubnub_http_input_cb() -> pubnub_http_inputcb(), pubnub_timeout_cb() -> pubnub_event_timeoutcb()
c-generic/libpubnub: Proper timeout handling

New pubnub_callbacks.timeout() call instead of .wait() parameters.