ENH: Color-filled Counties

This addresses what I consider somewhat of a corner-case when the user defines a "fill continents" color and then calls the "drawcounties" method. Currently, island portions of counties are drawn, but not filled in. This pull requests allows a user to supply a "facecolor" keyword argument that fills in the counties. Therefore, when specified, portions of counties that are (for example) islands results in the islands being filled.

This was done by converting the counties from a linecollection to a polycollection and then setting the facecolor argument. The default behavior remains the same (e.g., no filling).

Additionally, this will allow for industrious users to create choropleth maps by supplying lists of colors to fill counties with as the "set_facecolor" method of a polycollection will accept lists.