v0.4

+ Added support of GET_BARCONFIG and barconfig_update event and examples for them
+ Added getters form main and event sockets
+ Added i3ipc::get_version()

~ Calling i3ipc::connection::prepare_to_event_handling() is no more necessary
~ Using i3ipc::errno_error when calling c-functions instead of std::runtime_error
~ Logging-subsystem became public
~ Refactoring of CMakeLists.txt

- i3ipc::connection::get_file_descriptor() removed