added simplified method for making a 2D scatter plot

[cjfuller]

This is a bit of code that lets you make a scatter plot with sensible defaults in as little code as:

x = [0,1,2,3]
y = [0,1,4,9]
Plotrb::Simple.scatter(x,y)

Hash keyword args are also available to customize some of the plot style.

[zuhao]

Thanks @cjfuller! Very clever to make it short and sweet. 👍