Implement a Simple Moving Average Crossover Strategy

[ryanr08]

Add an algorithm to algorithms.py that implements the simple moving average crossover strategy. This won't necessarily be a super robust algorithm, but it will be a good baseline algorithm to compare against the others.

More info about the algorithm can be found here: https://www.investopedia.com/articles/active-trading/052014/how-use-moving-average-buy-stocks.asp