tutorial 4 compilation error

[BurningEnlightenment]

Travis currently fails to build the fourth tutorial:

gcc.compile.c++ ../../../bin.v2/libs/application/example/tutorial/gcc-4.6/debug/threading-multi/main_stage4.o
In file included from ../example/tutorial/main_stage4.cpp:12:0:
../example/tutorial/myfunctor_stage4.hpp:77:18: error: field ‘my_log_file_’ has incomplete type
../example/tutorial/myfunctor_stage4.hpp: In member function ‘int myapp::operator()()’:
../example/tutorial/myfunctor_stage4.hpp:36:7: error: ‘my_log_file_’ was not declared in this scope
../example/tutorial/myfunctor_stage4.hpp: In member function ‘bool myapp::stop()’:
../example/tutorial/myfunctor_stage4.hpp:48:7: error: ‘my_log_file_’ was not declared in this scope
../example/tutorial/myfunctor_stage4.hpp: In member function ‘bool myapp::pause()’:
../example/tutorial/myfunctor_stage4.hpp:58:7: error: ‘my_log_file_’ was not declared in this scope
../example/tutorial/myfunctor_stage4.hpp: In member function ‘bool myapp::resume()’:
../example/tutorial/myfunctor_stage4.hpp:67:7: error: ‘my_log_file_’ was not declared in this scope
../../../boost/asio/error.hpp: At global scope:
../../../boost/asio/error.hpp:258:45: warning: ‘boost::asio::error::system_category’ defined but not used [-Wunused-variable]
../../../boost/asio/error.hpp:260:45: warning: ‘boost::asio::error::netdb_category’ defined but not used [-Wunused-variable]
../../../boost/asio/error.hpp:262:45: warning: ‘boost::asio::error::addrinfo_category’ defined but not used [-Wunused-variable]
../../../boost/asio/error.hpp:264:45: warning: ‘boost::asio::error::misc_category’ defined but not used [-Wunused-variable]

    "g++"  -ftemplate-depth-128 -O0 -fno-inline -Wall -g -pthread -fPIC -m64 --coverage -std=c++98 -fno-strict-aliasing -DBOOST_ALL_DYN_LINK -DBOOST_ALL_NO_LIB -DBOOST_ALL_NO_LIB=1 -DBOOST_CHRONO_DYN_LINK=1 -DBOOST_DATE_TIME_DYN_LINK=1 -DBOOST_FILESYSTEM_DYN_LINK=1 -DBOOST_LIB_DIAGNOSTIC -DBOOST_PROGRAM_OPTIONS_DYN_LINK=1 -DBOOST_SYSTEM_DYN_LINK=1 -DBOOST_SYSTEM_NO_DEPRECATED -DBOOST_THREAD_BUILD_DLL=1 -DBOOST_THREAD_POSIX -DBOOST_THREAD_USE_DLL=1 -DDATE_TIME_INLINE  -I"../../.." -I"../../../../type_index" -c -o "../../../bin.v2/libs/application/example/tutorial/gcc-4.6/debug/threading-multi/main_stage4.o" "../example/tutorial/main_stage4.cpp"

...failed gcc.compile.c++ ../../../bin.v2/libs/application/example/tutorial/gcc-4.6/debug/threading-multi/main_stage4.o...

[BurningEnlightenment]

Fixed with #55.