Calculator (RPN) (Unreleased)

Rev 0.8.0
RPN calculator. It means dual operations are performed between last two operands in the stack
If just one operand is present calculator will raise an error message
15-digits precision
100 RPN registers
Basic operations: +, - *, /
Floating point. Fixed at four or zero digits, selectable

See wikipedia for RPN documentation
https://en.wikipedia.org/wiki/Reverse_Polish_notation

Example: (3 * (5+2)) = ->>
3 First operand
ENTER The first operand is entered. The second operand is initially duplicated, but then allow replacement
5 Second operand
ENTER The second operand is entered in the same way
2 The third operand
+ add operation of last two operand, 5 and 2. Operand now are 7 and 3
* The last operation is executed, multiplying 7 and 3, and resulting in 21. Now there is only one operand: 21

Know issues or improve opportunities:
* a button to remove last digit
* implement landscape option
* better documentation in Google play. And better graphics
* a link to provide direct email feedback
* Add some new functions. 1/x sqrt and others?

If you find something else, or any other comment, it will be very much appreciated.