Modified ggmaps section

[Mrugankakarte]

I have added examples for using stamenmaps with ggmaps.

[jtr13]

This looks great! As soon as I get a chance I will test the code. In the meantime, please add ggmap to the imports section of the DESCRIPTION file on your branch. Also, is it possible to read the uscities.csv from a URL? We had a problem with the repo size growing quickly so would rather not add large data files.

[Mrugankakarte]

I have added the ggmap to DESCRIPTION file. Regarding the data, unfortunately I cannot read it directly from url, but I have removed the csv file and replaced it with the zipped version which is 900Kb instead of 5Mb. I am directly read the csv file using unz().

[jtr13]

Let's find a time to discuss perhaps after class one day. I like the use of longitude and latitude for the military bases, but I find it problematic to plot density at points rather than areas. I'm not convinced the graphs are really showing density.

[Mrugankakarte]

As discussed I have changed the population density to spread of counties. Kindly let me know if any changes are required.