Bugs fixed

Implements:

• #1
• #2
• #3
• #4
• #5
• #6
• #7
• #8
• #9
• #10

Fixed bugs:

• #11

All unit test pass.

Application compiles without warnings.

Unit test compiles with 65 warnings in CalculatorBrainTests.swift, all related to the depreciated variables result, description and resultIsPending.