Update topology bbox when applying toposimplify or topoquantize

[mattijn]

This PR fix #117. Once toposimplify is used the bbox of the overall topology should be updated as well. This is done through this PR. One thing to consider are data files that both contain points and lines. A topology does not apply on points, so for computing the bbox the location of the points should also be considered.

Example that the bbox is updated once applying topoquantize:

import topojson as tp

# load example data representing continental Africa
data = tp.utils.example_data_africa()  
# compute the topology
topo = tp.Topology(data)  
# apply simplification on the topology and render as SVG
bbox = topo.topoquantize(10).to_dict()['bbox'] 

assert bbox == (0, 0, 9, 9)