Adds buttons on screen

Core.java creates and sets size and visibility to CalculatorGUI. Adds
buttons to CalculatorGUI.
Resizes buttons.

Sets panel p3's buttons size from 60x60 to 48x48 pixels.
Erases unnecessary argument.

Erases unnecessary JButton jbtEqual from class CalculatorGUI.
Merge pull request #5 from Jormatosijorma/devel

Adds buttons on screen