1.7: Basic memory handling and backspace ability

The calculator app has been reworked to provide the user with a more advanced tool; the changes include:
+ Memory addition and retrieval using `localStorage` API.
+ New indicator to show memory status and error in calculus.
+ New buttons to perform memory actions and backspace functions.
+ A higher-quality 128px (shown in splash screen).
+ Version bump to 1.7 in manifest and zip package.