Add dot icon for cities and star icons for capital cities

[zekefarwell]

Closes #24.

This PR adds the following icons for cities:

• A circle inscribed five pointed star for national capital cities (admin level 2)
• A plain five pointed star for state capital cities (admin level 4)
• A dot for other cities

I originally tried the circle inscribed star for state capitals and a slightly larger, double concentric circle inscribed red star for national capitals as this was a pattern I saw on a number of different US highway maps. However, these icons need to look good at a very small size and it was hard to fit enough detail into the national capital icon while also not looking blurry on a standard resolution screen. So I simplified them and I think the results look pretty good. Let me know what you think.

[ZeLonewolf]

These look great. It's amazing how the star-in-a-circle gives the map such an American feel. I tried playing with the zoom that the capital icons appeared at, and I was unable to make it look any better than you have it.