Completed extra credit 4 and 5

Implements:

• #1
• #2
• #3
• #4
• #5
• #6
• #7
• #8
• #9
• #10
• #11
• #12
• #13
• #14
• #15
• #16
• #17

All unit test pass.

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