Fix invalid geoms

[mattijn]

This PR fix #79.

geoms_bad = [
    {
        "type": "MultiPolygon",
        "coordinates": [[[0, 0], [1, 0], [1, 1], [0, 1], [0, 0]]]
    },
    {
        "type": "MultiPolygon",
        "coordinates": [[[0, 0], [1, 0], [1, 1], [0, 1], [0, 0]]]
    }
]

import topojson
tp = topojson.Topology(geoms_bad)
tp

WARNING:root:removed 2 invalid geometric objects
Topology(
{'arcs': [],
 'bbox': [],
 'coordinates': [],
 'objects': {'data': {'geometries': [], 'type': 'GeometryCollection'}},
 'type': 'Topology'}
)

Where the result still can be parsed to the to_geojson() function:

tp.to_geojson()

'{"type": "FeatureCollection", "features": []}'