add add_basemap function to new plotting.py

[ljwolf]

This does & documents #20.

A few things:

1. I started writing a plot_with_basemap(geodataframe, ...) function, but we'd have to decide how much to assume about the crs of geodataframe. I thought it'd be more convenient to just reproject anything that's not in webmerc, but there's support elsewhere for raw lat-long.
2. plot_map from place.py might be more properly put into plotting.py, or plotting.py should be named something else.

[coveralls]

Coverage decreased (-2.4%) to 89.888% when pulling 4850849 on ljwolf:master into 0b997d3 on darribas:master.

[coveralls]

Coverage decreased (-3.3%) to 88.95% when pulling 76ea396 on ljwolf:master into 0b997d3 on darribas:master.

[ljwolf]

OK, this is the simplest modification to address #20 through a distinct plotting function with automatic zoom.

This gives us 2 plotting functions,

1. plot_map, which works on places
2. add_basemap, which adds a basemap to the axis

I went a step further in my placeplot branch. There, I've stuffed plot_map into the Place class as a plot method, so Place('Austin, TX').plot() would plot Austin.

If we took this further step, it would make it so that there is one plotting function which adds basemaps to axes, add_basemap, but all places can be plotted using their .plot method.

[ljwolf]

This'll get superseded in a release PR