Remove a debug check in backend loop.
Adding -I. when compiling cc code.
Initial support for threaded networking.

Accepting connections in separate thread.
Removed fd6 support.
shutdown network threadpool on exit.
Add missing threadpool header.
Use evconnlistener on inbound connections.
Use bufferevent for user connections.

Introduce "tick" for virtual time and promote main evenloop.
Removed VA* macros.
Add add_realtime_event() to allow execute in main thread.

Add check for libevent_pthreads library.
Upgrade libstdc++ for travis build.
Clear-up heap allocations to silence valgrind.
Use clang-format to format source
Format using google style.
Include source revision in program name.
Merge pull request #180 from sunyc/master

Adding skeleton support for multithreaded networking subsystem.