Connected the signals of the help menu with the corresponding slots.
TODO: make the program useable for the command line.

 On branch master
 Changes to be committed:
	modified:   build-calculator-Desktop-Debug/calculator
	new file:   build-calculator-Desktop-Debug/help.html
	modified:   calculator/calculator.pro
	new file:   calculator/help_dialog.cpp
	new file:   calculator/help_dialog.hpp
	modified:   calculator/main_window.cpp
	modified:   calculator/main_window.hpp