c-generic/libpubnub: Make sure pubnub_connection_cleanup() is called before pubnub_connection_finished()

This allows finished_cb to issue another PubNub request, making it possible to easily callback-chain requests.
c-generic/libpubnub: Add libevent support as an alternative for the sync backend.
c-generic/libpubnub Makefile: Drop -DDEBUG by default; use make XCFLAGS=-DDEBUG to get debug version
c-generic/libpubnub Makefile: Split essential CUSTOM_CFLAGS to SYS_CFLAGS
c-generic/libpubnub pubnub_callbacks.unused: Extra item for forward binary compatibility
c-generic/libpubnub PUBNUB_API: New macro that now makrs all functions to be exported from the library
c-generic/libpubnub: Build an actual libpubnub.so, link example-* to it
c-generic: Check in my personal TODO file to track further progress