Finish 4 remaining exercises from Week 2

[cooke99]

• [x] 2-5 Fibonacci Number
• [x] 2-6 Last digit of sum of Fibonacci numbers
• [ ] 2-7 Last digit of sum of fibonacci numbes again
• [ ] 2-8 Last digit of sum of squares of fibonacci numbers

[cooke99]

Leaving off the last 2 exercises. Achieved 75% score on Week 2 with 6/8 exercises. Need to move on and continue with the rest of the course now.