I/O tools for database, shapefile, and sim framework

[daradib]

Major changes:

• TableFrame class for read-only DataFrame-like access to a database table.
• io.df_to_db function for exporting to database tables using PostgreSQL COPY.
• sim.plot function for plotting UrbanSim sim framework relationships using GraphViz.
• spatialtoolz.validate function for fixing invalid geometries using ST_MakeValid and support for it in spatialtoolz.conform_srids function.
• TableLoader.get_path method to return path to file in data directory.
• io.dbf_to_db function and TableLoader.get_attributes method to read shapefile attribute table in dBase format using GDAL.
• Rename index_name to index_col for consistency with pandas.

New functionality needs proper tests. Unfortunately tests will be deferred to next year because of time constraints.