Added option for alternative tz_world.json file

Since the construction of the polygons takes a long time it is helpfull
to provide an option to pass an alternative, smaller, simpler file to be
used as the basis.

Since the polygones are global, the test need to run serialized now.
updated readme
Merge pull request #5 from sideshowcoder/make_tz_file_configurable

Thanks to sideshowcoder you can now pare down the database if you prefer.  Or I suppose replace the timezones with arbitrary other shape boundaries.