Adding plotting for points/lines, support for Fiona filter, legend labels in plots, use of arbitrary axes objects

I decided to jump in and try and use GeoPandas for some litigation work I am doing. To get done what I needed end-to-end, I added the following things to GeoPandas that will probably be useful in some form to others in the long run:

* Plotting for points and lines (I have not tested the geopandas/shapely operations on these geometries)
* Labeling scheme so that legends can be used in plots (should probably also add an option that uses the GeoDataFrame or GeoSeries index (or other column) if the user desires)
* Support for the Fiona collection filter by bbox in the GeoDataFrame.from_file open call
* Ability to specify axes to use in the .plot() calls, instead of creating new automatically

As part of this work I also was starting with some Pandas DataFrames that we're really trajectories, so I wrote some middlewear that converts each DataFrame into an individual shapely geometry (lines in this case, but points is probably also useful) and then packs them all into GeoSeries and GeoDataFrames. Would it be useful in GeoPandas to include some kind of utility methods that converts a sequence of DataFrames into geometries or GeoPandas objects given some user inputs with regard to what columns to use for x, y[, z]?

Cheers,
Alex