New Exercise: Exponential Growth and Decay Word Problems

This exercise addresses [Trello card 461](https://trello.com/card/exponential-growth-word-problems/4d87e664967a0775082939ab/461), and is intended to slot in after the [Word Problem Solving - Exponential Growth and Decay video](https://www.khanacademy.org/math/trigonometry/exponential_and_logarithmic_func/exp_growth_decay/v/word-problem-solving--exponential-growth-and-decay). No knowledge of logarithms required.

Contains 5 types of problems, including the following 2 from the video:
![exponential-growth-2 169](https://f.cloud.github.com/assets/238458/251751/2cdf95ae-8b72-11e2-88ae-b0059a4a4e31.png)
![exponential-decay-1 118](https://f.cloud.github.com/assets/238458/251748/23de963a-8b72-11e2-8b80-364c7ef6f87a.png)